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 64041f0e6e.
lookup.tests.LookupTests.test_regex_non_string fails under Postgres.
We should also try to rewrite the test using an existing model.