Flaky tests because CMOR tables have not been loaded

[bouweandela]

These tests occasionally fail because the CMOR tables have not been loaded before they are run:

FAILED tests/unit/preprocessor/_derive/test_soz.py::test_soz_calculate_hybrid_plevs - KeyError: 'plev19'
FAILED tests/unit/preprocessor/_derive/test_toz.py::test_toz_calculate_masked_cubes_hybrid_plevs - KeyError: 'plev19'
FAILED tests/unit/preprocessor/_derive/test_troz.py::test_troz_calculate_hybrid_plevs - KeyError: 'plev19'
FAILED tests/unit/preprocessor/_other/test_other.py::test_align_metadata - ValueError: Unable to convert from 'Unit('celsius')' to 'Unit('unknown')'.
FAILED tests/unit/preprocessor/_other/test_other.py::test_align_metadata_alt_names[sic] - ValueError: Unable to convert from 'Unit('%')' to 'Unit('unknown')'.
FAILED tests/unit/preprocessor/_other/test_other.py::test_align_metadata_alt_names[siconc] - ValueError: Unable to convert from 'Unit('%')' to 'Unit('unknown')'.
FAILED tests/unit/preprocessor/_other/test_other.py::test_align_metadata_invalid_short_name_strict - AssertionError: Regex pattern did not match.
  Expected regex: "Variable 'zzz' not available for table 'Amon' of project 'CMIP6'"
  Actual message: "Unable to convert from 'Unit('1')' to 'Unit('unknown')'."
FAILED tests/unit/preprocessor/_other/test_other.py::test_align_metadata_invalid_short_name_not_strict - ValueError: Unable to convert from 'Unit('1')' to 'Unit('unknown')'.

Example runs:

• https://app.circleci.com/pipelines/github/ESMValGroup/ESMValCore/14951/workflows/445e2fc0-5fa3-4d21-84bc-da299a18b623/jobs/60905/tests
• https://app.circleci.com/pipelines/github/ESMValGroup/ESMValCore/14952/workflows/0bb0ef3a-f12c-40ce-a265-e1c8b839360f/jobs/60908/tests

[bouweandela]

We do automatically set up the default configuration for the tests:

ESMValCore/tests/conftest.py

Lines 28 to 55 in e232092

	@lru_cache
	def _load_default_config():
	"""Create a configuration object with default values."""
	with warnings.catch_warnings():
	warnings.filterwarnings(
	"ignore",
	message="Do not instantiate `Config` objects directly",
	category=UserWarning,
	)
	cfg = Config()
	cfg.load_from_dirs([])
	return cfg
	@pytest.fixture
	def cfg_default():
	"""Create a configuration object with default values."""
	cfg = _load_default_config()
	return deepcopy(cfg)
	@pytest.fixture(autouse=True)
	def ignore_existing_user_config(
	monkeypatch: pytest.MonkeyPatch,
	cfg_default: Config,
	) -> None:
	"""Ignore user's configuration when running tests."""
	monkeypatch.setattr(CFG, "_mapping", cfg_default._mapping)