mir_build: Rename `TestCase` to `TestableCase`
In the spirit of rust-lang/rust#149946, this is another renaming of something I've always found confusing.
When lowering match conditions, there is a subtle distinction between the kind of *test* being performed (on a scrutinee place), the possible outcomes of that test, and the different “cases” (corresponding to pattern nodes) that the test is distinguishing. Cases imply a particular preferred test, but once a test is chosen it can also interact with other cases as well.
I have often mixed up `TestKind` and `TestCase`, since the names are so similar, and they share several variant names. Therefore, this PR renames `TestCase` to `TestableCase`, to emphasise that these are the things selected by whatever test is being performed.
There should be no change to compiler behaviour.
Matthias Krüger
7180fd1659