Fix intermittent test failure in paltest_namedmutex_test1

Fix intermittent test failure in paltest_namedmutex_test1

Fixes dotnet/coreclr#5169:
- Child thread/process was not closing their test mutex before indicating completion to the parent thread/process
- Fixed by enclosing the actual child test code in braces to force closing before the cleanup code runs. This is a minimal fix for the issue, I have a separate change that generally improves and refactors the tests further.

Commit migrated from https://github.com/dotnet/coreclr/commit/f60d720fce00189236fcf7f171b8aa5aece618d7