New tests for `parallel-tests'. * tests/parallel-tests.test: New file, to expose a bunch of features of `parallel-tests': VERBOSE, clean, TEST_SUITE_LOG, test dependencies, DISABLE_HARD_ERRORS. * tests/parallel-tests2.test: New file, test check-html. * tests/parallel-tests3.test: New file, test concurrency. * tests/parallel-tests4.test: New file, test suffix rules. * tests/parallel-tests5.test: New file, demonstrate compile/test concurrency. * tests/defs.in: Add a `required' check for rst2html. * tests/Makefile.am: Update. Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>