Update Travis to also run the test suite

There are several (8) directories whose tests either fail to compile (btrfs, devmapper) or fail to run inside Travis for various reasons (of which I've tried to document at least somewhat in `.travis.yml` directly).  We don't really have a way to do `TESTDIRS_NOT` or something like that, so I resorted to just deleting `*_test.go` from the offending directories (which of course could be made even more specific over time to only delete specific offending test files so that we could still test the rest of the package minus the failing tests).

cc @unclejack @tiborvass
See https://travis-ci.org/tianon/docker/jobs/34445725#L401 :smile: