New tests for `parallel-tests'.

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>