fix: flaky PollWatcher tests

[riberk]

This PR fixes tests for a PollWatcher.

I think, the test failed due to races between FS and our code.

It might be the behavior described below

• The test creates a file
• The test watch the parent
• The PollWatcher scans the parent and found an initial state of the file
• The test removes the file
• The test waits for the file to be removed (by fs calls)
• The test calls detect_changes
• The watcher calls read_dir and still find the file in the dir (because the directory info has not been updated yet, probably?)
• The watcher raises Modify instead of Remove
• The test fails

I added the two helper methods to wait a directory state - sleep while / until a child exists in a read_dir call results and called them in any suitable situation

closes #731