[llvm-cov] Change some FileCheck prefixes to make tests reusable (NFC)

I'm planning on extending these two tests with checks that validate
html coverage reports. Make it easier to extend them by not using a
prefix called "CHECK".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274143 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed