Separate environment-dependent tests in Rakefile

+ Testing test suites is hard.
+ Update test globs in Rakefile: default "test"
  task excludes failing .rb test fixtures.
+ Extract environment-dependent tests into a
  separate Rake task. Exclude these from default
  tests so they don't fail Travis builds.
Check for valid file on TestSuite initialization.

+ Fix TestSuite to raise an exception (caught by
  the CLI) if an invalid file is passed to the
  test or submit CLI commands.