Don't watch the whole directory in the file watcher test

When watching the whole test directory we'll also get some events for
the directory, and those events can come with different order depending
on the OS, so we can't rely on them. Instead, we should watch a specific
file, so we can control the order of the events.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

tests/test_file_watcher_integration.py

@@ -26,7 +26,7 @@ async def test_file_watcher(tmp_path: pathlib.Path) -> None:
     number_of_writes = 0
     expected_number_of_writes = 3
 
-    file_watcher = FileWatcher(paths=[str(tmp_path)])
+    file_watcher = FileWatcher(paths=[str(filename)])
     timer = Timer(timedelta(seconds=0.1), SkipMissedAndDrift())
 
     async for selected in select(file_watcher, timer):