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

[Fangrui Song via llvm-commits]

MaskRay wrote:

(The previous title "[unittest] Add lit option to allow disabling sharding in unittest") was in the description, so commit 6eee238975e435ad3e5aa7f2b1817ca756911e04 got a duplicated title.)

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