Avoid slow database serialization when possible

[bluetech]

I have a large project where the serialize_db_to_string function from Django takes around 5s on every call to pytest. This DB has some pre-loaded fixtures so it doesn't start the test run empty.

The relevant Django code is:

https://github.com/django/django/blob/02a5cbfe76382da2a0414df17017185be5bd47f9/django/test/utils.py#L231-L239

This is called from pytest-django django_db_setup session fixture:

pytest-django/pytest_django/fixtures.py

Lines 144 to 148 in b732869

	db_cfg = setup_databases(
	verbosity=request.config.option.verbose,
	interactive=False,
	**setup_databases_args,
	)

We don't pass any of the serialize parameters so it defaults to serializing.

The Django DiscoverRunner doesn't always serialize:

https://github.com/django/django/blob/02a5cbfe76382da2a0414df17017185be5bd47f9/django/test/runner.py#L1081-L1089

It only serializes if there are any tests in the suite which use serialized_rollback:

https://github.com/django/django/blob/02a5cbfe76382da2a0414df17017185be5bd47f9/django/test/runner.py#L1044-L1056

We should try to do the same if possible.