changefeedccl: run schema tests under both schema changers

At the moment, we run some tests only under the legacy schema changer (which will soon be moved over to the new, declarative schema changer https://github.com/cockroachdb/cockroach/issues/106906) or only under the declarative schema changer.

Ideally, we should metamorphically decide which schema changer tests run under. The declarative schema changer is enabled in production, so we should probably use it 90% of the time.

This is a little bit tricky because some tests need to change their assertions when changing schema changers (see https://github.com/cockroachdb/cockroach/pull/107184). In this case, it would be ideal to bifurcate those tests.

Jira issue: CRDB-30099

Epic CRDB-9179