Stabilize QFileSystemWatcher test.

Stabilize QFileSystemWatcher test.

- Run each test in a temporary directory, avoid writing test
  files in source/build tree and prevents tests being influenced
  by left-overs from previous runs and locks of the application
  on the current directory.
- Modify test to be able to use absolute paths to the temporary
  directory.
- Skip parts of test removeFileAndUnWatch if a race condition
  occurs.

Task-number: QTBUG-24029
Change-Id: I215cc2e0fe6f92d2ffe597b01cdc9c9a39e3c5b4
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>