FulltextSearchableTest doesn't clean up after itself.

The extension doesn't get unloaded correctly at the end of the test,
resulting in tests afterwards sometimes failing because the table
type is reset back to InnoDB.

See silverstripe/silverstripe-cms ed8ee4e9b for a similar fix done
in the cms module.
Merge pull request #2576 from halkyon/fulltextsearch_test

FulltextSearchableTest doesn't clean up after itself.