automake: new option 'serial-tests'

automake: new option 'serial-tests'

Currently, automake offers a 'parallel-tests' option to enable the
use of the parallel testsuite harness in the generated Makefiles,
but no option to explicitly state the intention of using the older
serial testsuite driver (which is currently the default).

This makes the parallel test harness seems like a second-class
citizen (while actually it should be the other way around); more
importantly, it will prevent us from making the parallel harness
be the default one in future automake versions, since the users
of the serial harness would then have no way to instruct automake
to continue using it.

We solve all of this by introducing a new 'serial-tests' option.

* lib/Automake/Options.pm (_process_option_list): Recognize the
new option, and related minor refactoring.
* tests/serial-tests.test: New test.
* tests/list-of-tests.mk: Add it.
* doc/automake.texi: Update.
* NEWS: Likewise.