[llvm] [unittest] Add option to allow disabling sharding in unittest (PR #67063)

[Joe Loser via llvm-commits]

https://github.com/JoeLoser approved this pull request.

Thanks for tackling this - it may be useful for those running into https://github.com/llvm/llvm-project/issues/56492 or derivative issues.  I ran into that issue in the past a year ago.

https://github.com/llvm/llvm-project/pull/67063