Ensure clean-up for func tests

[dashmage]

In the test_config_changed_port and test_config_changed_log_level tests, the clean-up steps

await app.reset_config(["exporter-port"])

may not execute in case of a failure in any of the previous lines of code.

In order to fix this, we would need a cleanup phase where even if an error occurs, the cleanup will happen before the test exits.

[dashmage]

The addCleanup method could be used for this purpose if we define the TestCharm as a TestCase.