test: use unique file names in fs trace test
Should fix test flakiness that is presumably caused by the asynchronous nature of the unlink operation on Windows.
It's been observed that sub-tests randomly fail with "permission denied" errors when trying to create a new file in a directory with appropriate permissions.
The DeleteFile() NT API call makes a file inaccessible and marks it for deletion but doesn't actually delete it until the last open handle has been closed. Accessing such a file fails with ERROR_ACCESS_DENIED.
Processes can close handles manually or wait for the operating system to close them asynchronously after process termination. I speculate it's the latter that's causing the test to turn flaky.