[th/rename-tests-with-word-fail] rename tests to not contain the word "fail"

When I look at the test results (in the web page), the first thing thing I do is CTRL-F "fail". That finds a lot of irrelevant occurrences, because the term "fail" is part of the names of tests.

That is annoying. Rename those tests.

Since I don't have a better idea, use an intentional misspelling "faiil" instead.