Tests: Make PsqlDosStorage profile unload test more robust (#6115)

The `test_unload_profile` test verifies that if a loaded profile is
unloaded, it properly relinquishes of the session that is maintained by
sqlalchemy. It did so by checking that after unloading, there were no
sessions being referenced. However, this would fail sometimes, because
another session may still be held on to, even though that session had
nothing to do with the test.

A more robust test is simply to check that after unloading, there is
exactly one less session being held on to.

Cherry-pick: 1c72eac

tests/storage/psql_dos/test_backend.py

@@ -147,16 +147,23 @@ def test_unload_profile():
 
     This is a regression test for #5506.
     """
+    import gc
+
     from sqlalchemy.orm.session import _sessions  # pylint: disable=import-outside-toplevel
 
+    # Run the garbage collector to ensure any lingering unrelated sessions do not cause the test to fail.
+    gc.collect()
+
     # Just running the test suite itself should have opened at least one session
-    assert len(_sessions) != 0, str(_sessions)
+    current_sessions = len(_sessions)
+    assert current_sessions != 0, str(_sessions)
 
     manager = get_manager()
     profile_name = manager.get_profile().name
 
     try:
         manager.unload_profile()
-        assert len(_sessions) == 0, str(_sessions)
+        # After unloading, the session should have been cleared, so we should have one less
+        assert len(_sessions) == current_sessions - 1, str(_sessions)
     finally:
         manager.load_profile(profile_name)