llvm-profdata: Clean up and reorganize some tests

This moves some tests around to make it clearer what's being tested,
and adds very rudimentary comment syntax to the text input format to
make specifying this kind of test a little bit simpler.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214235 91177308-0d34-0410-b5e6-96231b3b80d8