Refactor testtex

* Eliminate the code paths for testing "batch" texture functions, which
  are officially deprecated anyway and made the test code much more
  cluttered.
* Use parallel_image to run the texture tests in a multithreaded fashion,
  both so that we can benchmark thread scalability, as well as ensure that
  every time we run a testsuite, we are verifying thread safety of the
  texture system.