[llvm-cov] Disable threading in a test. NFC.

[llvm-cov] Disable threading in a test. NFC.

PR30735 reports an issue where llvm-cov hangs with a worker thread
waiting on a condition, and the main thread waiting to join() the
workers. While this doesn't appear to be a bug in llvm-cov or the
ThreadPool implementation, it would be helpful to disable the use of
threading in the llvm-cov tests where no test coverage is added.

More context: https://bugs.llvm.org/show_bug.cgi?id=30735

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307610 91177308-0d34-0410-b5e6-96231b3b80d8