[test] Clean up previous raw profile before merging into it

This fixes a test failure in instrprof-set-file-object-merging.c which
seems to have been caused by reuse of stale data in old raw profiles.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@372041 91177308-0d34-0410-b5e6-96231b3b80d8