[llvm-cov] Repair a test. NFC.

The checks with the MARKER prefix were not being run over the right
input, because stderr was not redirected properly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313596 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed