[BLD] Run 'full' test suite on Travis, including 'slow' tests

There's no reason *not* to run the full test suite on Travis, so long
as we fit in under the time limit. And no way to find out if we fit
under the time limit except to try ;-).
Merge pull request #3200 from njsmith/travis-full-tests

[BLD] Run 'full' test suite on Travis, including 'slow' tests