Add a more versatile test chooser

So far, the test runner (test/run_tests.pl) could get a list of tests
to run, and if non were given, it assumes all available tests should
be performed.

However, that makes skipping just one or two tests a bit of a pain.
This change makes the possibilities more versatile, run_checker.pl
takes these arguments and will process them in the given order,
starting with an empty set of tests to perform:

    alltests            The current set becomes the whole set of
                        available tests.
    test_xxx            Adds 'test_xxx' to the current set.
    -test_xxx           Removes 'test_xxx' from the current set.  If
                        nothing has been added to the set before this
                        argument, the current set is first initialised
                        to the whole set of available tests, then
                        'test_xxx' is removed from the current set.
    list                Display all available tests, then stop.

If no arguments are given, 'alltests' is assumed.

Reviewed-by: Emilia Käsper <emilia@openssl.org>