Overhaul of tests/README. * tests/README: General overhaul. Mention parallel test suite execution, the test-suite.log file; document `Exit' vs. `exit', `defs' vs. `defs-p', the naming of `parallel-tests' tests. * README: Simplify, point to tests/README. Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>