Add option to run-test.sh to allow skipping troublesome tests.

Description of the need

Some tests fail when run in certain environments that don't precisely match the CI runner. To help filter out the noise when running tests locally, it'd be nice if we could specify tests to skip.

Proposed solution

Add a --skip option to run-tests.sh and have it look at a .simpletestignore file to get a list of tests that should not be run locally.