Integrate bin-test/test with Gradle

SOLR-11749 created a regression test suite for our bin/solr scripts, and SOLR-12137 expanded this coverage to Windows as well.

Now that we have a test suite on both Windows and *nix, we should create an ant target allowing devs to easily trigger these tests. We should also consider whether there's community consensus behind adding these tests to existing runs of ant test. If there are concrete but fixable things preventing ant test integration, we should fix those.