Being Moral is an end goal.

But moral people do go to parties whilst they might have an infection.

Speak for yourself.  Sure, in a vacuous sense, anyone might have an infection at any time, but what we're talking about here is "whilst they have good reason to think they might have an infection".  For me, even before COVID, I've had a general policy of not seeing people when I'm sick, and stayed home from some parties I otherwise would have enjoyed.  At least for me, a sore throat would be rather unusual if I weren't sick, so that would normally be enough evidence for me (unless it was so slight I wasn't sure it was a real symptom; or if I'd overused my voice or had some other non-sickness plausible cause).  If I had a magical test that was free and always told me correctly whether I had an infectious disease, that would be wonderful and I'd use it a lot.

A sensible policy would take into account your estimated probability that you're infectious, and the number of people you'd likely infect.  Also the expected enjoyment you'd get from the party, and the expected negativity an infection would inflict on other people.  (Note that there are circumstances, such as Omicron COVID, where one might reasonably say "If they don't catch it from me, there's still a >80% chance they'll catch it elsewhere", which may downgrade the "expected negativity" estimate.)  Maybe a scaling factor for how you care less about other people than about yourself—I think it's reasonable to have that, and for "people you'd encounter at a party" I think it should normally be between 0 and 1.  I'm not exactly a utilitarian, but I would say that, among friends and people you cooperate with, it generally makes the most sense for policies to be based on utilitarian calculations.

You say that a bunch of people do in fact behave in the manner you've described.  I can only chalk this up to "a bunch of people are disappointing".  Or—along the lines of what TAG says, in this particular case, I suspect that these people have encountered official rules that say "if you test positive, then you must stay home"—and while I'm not sure how common it is for party hosts to explicitly state such rules, these people have internalized the rule and are trying to obey it; but I would say that either (a) they don't fundamentally care about not infecting other people, only about not disobeying those rules (and getting caught?), or (b) they haven't thought things through.  Either seems plausible to me.

Or, I dunno.  Could they have been confident they don't have COVID but worried about a false positive?  Doesn't sound like it, though.

The general pattern of avoiding potentially hedonically harmful knowledge is indeed widespread. This includes your example of "this particular action I want to take could be immoral if I could evaluate its consequences". Other examples are the general flinching from things like checking your weight or sugar or going to see a doctor, or paying bills or...

Giving a made-up (though believable) story does not justify expanding to what "we" care about.  Ways in which humans are irrational and self-deceiving are many, but that doesn't mean they don't ALSO care about others' feelings and experiences.

Doesn't society already consider it immoral to go to crowded places untested when you suspect you have COVID? This is not just about a specific detail of this specific story - one important feature is morality is preventing humans from convincing themselves that the thing they want to do is the utilitarian choice [? · GW]. We decided that going untested is immoral precisely because people like Alice who avoid testing themselves for such reasons.

Instead of morality, I think what Alice seeks here is deniability. If Alice does not take the test, she can convince herself (and possibly others?) that the probability of her sore throat indicating COVID is as low as she wants. No one else can really tell how bad it was - certainly not days later, when the thing is discovered - she may even claim that she felt nothing unusual. She is still immoral, but she can at least convince herself that she has done nothing wrong.

You don't just get -50 if you are sick and you go. It's also negative if you go and someone else who is sick goes.

This is a very common behaviour, I agree, but I am not really sure the simplified explanation is useful for understanding anything (I know you are specifically pointing out in the last paragraph that you are perfectly aware that your explanation is a simplification). 

I think unfolding things a bit we get a much better idea of what's really going on:

1. Alice is a human being, with a mind composed of a society of competing modules
2. Some of her modules do care about being moral. Some others care about having fun. Some others care about people. Some modules agree with each other, some others contradict each other. 
3. The society of modules carries some sort of internal voting, that results in Alice taking an action (e.g. going to the party). Not all modules have always the same strength. They are heavily influenced not only by external factors but also internal factors, like hormonal levels
4. One specific module makes a narrative about why it is justified going to the party and that's the correct thing to do

Reducing Alice to a single entity is a convenient and yet wrong way of thinking of people and motivations

People care about breaking rules because there are punishments for breaking rules , both formal and informal. If people were unselfishly rational, and moral as a result of being unselfishly rational, there would be no need for punishments to shape behaviour. As it is, rules need to be formulated on the basis that the rule alone generates the desired behaviour, not on the basis that the individual gets the point of the rule.