Streamline the mentions of grep and FileCheck in TestingGuide.rst

grep is now only mentioned once in a sentence that explicitly says it's
deprecated. For FileCheck, there's no reason to repeat part of the
documentation that exists in CommandGuide/FileCheck.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172835 91177308-0d34-0410-b5e6-96231b3b80d8