This switches to generating per-file targets such as `//executable_semantics/testdata:tuple/equality_false.carbon.test`, instead of one test covering all files. Essentially this relies on bazel instead of lit to handle test parallelization.
I think lit_test accrued a bit of cruft as I switched approaches... I still may go further, but this is particularly fixing a bug where `FileCheck` (instead of `%{FileCheck}`) should've failed in tests.