runtests.pl: add '--skip <test>' option.

Hi EnsEMBL team. This pull request is a pre-requisite for the work I've been doing to get Travis-CI to run the test suite against SQLite as well as MySQL.

Since there are a couple of tests which are currently unsupported under SQLite, runtests.pl needs to be told to skip them. (Notably schema.t and schemaPatches.t.)

Thanks
Michael