Change scope of getting FD count in unit test fixture

This commit changes the scope for getting the count of open file
descriptors for our copytree tests. This gives more accurate
view of where an fd may leak during the test runs and eliminates
a potential area for false-positives.

tests/unit/test_copytree.py

@@ -129,7 +129,7 @@ def get_fd_count() -> int:
     return len(os.listdir('/proc/self/fd'))
 
 
-@pytest.fixture(scope="module")
+@pytest.fixture(scope="function")
 def fd_count() -> int:
     return get_fd_count()