Update tests to pass under GETTEXT_POISON (a mechanism to ensure
that output strings that should not be translated are not
translated by mistake), and tell TravisCI to run them.

* ab/fix-poison-tests:
  travis-ci: add job to run tests with GETTEXT_POISON
  travis-ci: setup "prove cache" in "script" step
  tests: fix tests broken under GETTEXT_POISON=YesPlease