[llvm] [llvm-lit] Precommit Tests for implementing the `unset` command in lit internal shell (PR #104618)

[James Henderson via llvm-commits]

jh7370 wrote:

Related to my most recent comment in the RFC, how many tests make use of `unset`, and which tests are they? My gut is that this functionality is unlikely to be needed at all, and a simple bit of rewriting of tests would be preferable, but if the number of tests si high, my view may change.

https://github.com/llvm/llvm-project/pull/104618