[llvm] [lit] Add LIT_CURRENT_TESTCASE environment variable when running tests (PR #168762)

[Andrew Haberlandt via llvm-commits]

ndrewh wrote:

@boomanaiden154 I added a testcase.

Apparently `env` (with no args) does not correctly chain with `| FileCheck %s`, so every testcase in `env-no-subcommand.txt` is broken (trivially passes no matter the output). I don't know how to fix that easily, so I'm just going out to bash here for this test.

https://github.com/llvm/llvm-project/pull/168762