For better integration with the build system and nicer output, use
Automake's TAP driver and gtester's TAP output mode.

There are a few things going on in this commit:

- We don't put logs in test_user_data/logs anymore. That really assumed
  in the first place that all tests would be run serially, and we want
  them to be run in parallel. So we'll let Automake take care of the
  stderr and debug outputs and redirect them to a per-test log file.

- We use the significant-to-Automake variable TESTS instead of
  TEST_PROGS. TESTS contains whatever is in check_PROGRAMS plus whatever
  else we put into it (our test script.)

- TESTS_ENVIRONMENT is also significant to Automake, but should not be
  set within Automake, instead the AM_TESTS_ENVIRONMENT variable should
  be used so that the developer can override TESTS_ENVIRONMENT from the
  command line. This variable consists of a sequence of commands and
  needs to end with a semicolon. (So we can stuff XVFB_START in there
  too.)

- We need to put the test invocation in a separate script since gtester
  doesn't pass the --tap option on to its test binary.

https://bugzilla.gnome.org/show_bug.cgi?id=775205