[Tests] Rename some test files for consistency

Most are named "atomic-something" so rename the few which were "atomic_something".  I keep typing the wrong name due to the inconsistency.  :)



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353956 91177308-0d34-0410-b5e6-96231b3b80d8