Fixed #23727 -- Inhibited the post_migrate signal when using serialized_rollback.

When using a TransactionTestCase with serialized_rollback=True,
after creating the database and running its migrations (along with
emitting the post_migrate signal), the contents of the database
are serialized to _test_serialized_contents.

After the first test case, _fixture_teardown() would flush the
tables but then the post_migrate signal would be emitted and new
rows (with new PKs) would be created in the django_content_type
table. Then in any subsequent test cases in a suite,
_fixture_setup() attempts to deserialize the content of
 _test_serialized_contents, but these rows are identical to the
rows already in the database except for their PKs.  This causes an
IntegrityError due to the unique constraint in the
django_content_type table.

This change made it so that in the above scenario the post_migrate
signal is not emitted after flushing the tables, since it will be
repopulated during fixture_setup().