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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

I don't think this is helpful to document here. It's hand-wavy ("test failures from an old compiler" - how, dependencies are in the monorepo). It's overly specific and not how dependencies work. I don't think we need to spell out merging in dependency order 

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