[llvm] [llvm][docs] Update TestingGuide with LLVM PR merge advice (PR #207754)

[John Otken via llvm-commits]

jotken wrote:

In this case "old compiler" means more than a day old.  The brief point I was trying to convey is don't merge PRs for llvm-project and test-suite on the same day.

https://github.com/llvm/llvm-project/pull/207754