When custom lookups were added, converting the search lookup to use
the new Lookup infrastructure wasn't done.
Some changes were needed to the added test, main change done by
committer was ensuring the test works on MySQL versions prior to 5.6.
This reverts commit 64041f0e6e7a773574d7cde978d16305200004ea.
lookup.tests.LookupTests.test_regex_non_string fails under Postgres.
We should also try to rewrite the test using an existing model.
The recent improvements should make shared_models obsolete.
This reverts commit 1059da8de675442e84381d6366c0be254681753e, reversing
changes made to 4fa7f3cdd9bcf50ec4c7f64a31c1dfc02c375f46.