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

[Matt Arsenault via llvm-commits]

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@@ -266,6 +266,10 @@ Put related tests into a single file rather than having a separate file per
 test. Check if there are files already covering your feature and consider
 adding your code there instead of creating a new file.
 
+If new tests depend on a separate LLVM pull request, wait at least a day after
+merging the LLVM PR before merging the test-suite PR.  This avoids test
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arsenm wrote:

Oh, this is about llvm-test-suite. This needs to spell that out, with a link to that repo 

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