[Documentation] Add guidelines for grouping tests together.

This was considered a good practice but it was not documented. Now it is.

Differential Revision:	http://reviews.llvm.org/D14733


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253281 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed