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

[via llvm-commits]

Harini0924 wrote:

@jh7370 After reviewing the number of tests that make use of the `unset` command, I found that there are only two tests (`afl-driver-stderr.test` and `afl-driver-close-fd-mask.test`). Given the low number, I decided to rewrite these tests rather than implement the `unset` command within the lit internal shell. This PR (#104880) reflects that decision.

However, this PR still checks the current state and will display an error message that guides users to use `env -u` instead of `unset` to unset environment variables. I believe this approach will be helpful for users by providing clear instructions on how to handle this situation correctly.

What are your thoughts on this patch? 

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