[XRay][compiler-rt][NFC] Clean up xray log files before running test

Improves the test behaviour in the face of failure. Without this change
the fdr-single-thread.cc test may leave around artefacts of a previous
failing run since the cleanup doesn't happen if any of the intermediary
steps fail.

Non-functional change.

Subscribers: llvm-commits
llvm-svn: 316548