A sh*t test is when something bad happens that she didn't want to have happen and she then gets to observe your reaction (and is keeping score the way you do during an NCAA Football game, and making more observations than Jeff Sagarin). This is as opposed to something bad happening that she wanted to have happen, in which case it's called a blowoff.
Some examples of sh*t tests:
1) She tells you "I just got my period" after you make definite plans. If you reply "Ok well then some other time..." you not only fail, she might hate you. You have to say "You are welcome to still visit, etc." and then she'll love you because she knows it's not just about sex to you (and half the time they can do it anyway, especially after a bath).
2) She cancels plans for a legitimate reason. Ideally she offers you a raincheck and says "please don't be mad" etc., but either way you can never react. You should say "Great, I needed time for (x and y) - see you (at other time)" or at least "np". Always reply to let her know you got her message. Spitefulness of any kind will destroy your chances. She shouldn't do this twice in a row but provided she offers a raincheck there is no limit. It's when she cancels with no raincheck (even once) that it's over.
3) She tells you she has a disease. You have to thank her for being honest, remember there is a chance you were asymptomatic and actually gave it to her, and get tested. Never criticize her for it or it encourages her to lie. Never bring it up again, it's only a negative subject. If you do refer to it, keep it lighthearted. This can win her over big time because the other guys will have said something mean, just hope it was curable.
4) She tells you she's not interested anymore. Women will often push you away because of how they feel in the short term, and ultimately use that to measure how much you mean to her. The key is to have no reaction. Just say "No problem" and go NC until she contact you or 60 days go by, if you are still interested (but only if you improved yourself as lack of attraction is always the root of this sh*t test).
In these 4 examples you find yourself at a crossroads and you either win her respect or lose her respect, there is no standing your ground. Though that's the nature of relationships anyway - her interest level is always rising or falling.