testsuite: avoid generating `*-p.test' tests, use a wrapper script

testsuite: avoid generating `*-p.test' tests, use a wrapper script

The generated `*-p.test' tests had already become just thin layers
around the corresponding test scripts.  This change makes the final
step, converting to the use of a generic wrapper script and thus
avoiding the extra test generation (similarly to what is done for
the `*.instspc' and `*.depmod' tests).

* tests/parallel-tests.sh: New file, driver script to run checks
the on the `parallel-tests' semantics by wrapping tests that use
the generic "Simple Tests" driver.
* tests/gen-parallel-tests: Update, mostly to reflect the new
`.ptest' extensions used for tests in $(parallel_tests).
* Makefile.am (TESTS_EXTENSIONS): Add `.ptest'.
(PTEST_LOG_COMPILER): Define, it calls `parallel-tests.sh'.
($(parallel_tests)): Do not really generate `*-p.test' tests
anymore; this is now just a dummy dependency declaration required
in order to have make actually produce expected log files from
the `.ptest.log' suffix rule.
(EXTRA_DIST): Distribute `parallel-tests.sh'.
(MAINTAINERCLEANFILES): Don't remove the `$(parallel_tests)', it
is not necessary anymore.
(generated_tests): Variable definition removed.
(TESTS): Update, by listing `$(parallel_tests)' directly instead
of `$(generated_tests)'
(expected_list_of_tests): Remove `$(generated_tests)'.
(maintainer-check-list-of-tests): No need to explicitly depend on
`$(expected_list_of_tests)' anymore.