Set ``settings.HAYSTACK_CONNECTIONS['default']['URL']`` back to its 'fac...

...tory settings' in ``solr_tests.FailedSolrSearchBackendTestCase.tearDown``, so that a test failure there will not impact all following tests due to the altered URL.

I ran into this problem when trying to get the entire test suite to pass, and this test case failed. Because it alters the Haystack URL in memory, then sets it back after all assertions, if any assertion failed, it would never get to the "sets it back" part. Every test afterward would then try to ping ``/solr/test_default/foo/``.