Objective-C++: Parallelize the tests.

Although this is a small testsuite, waiting for it to complete
when debugging Objective-C issues is unproductive.  Keep the number
of parallel tests to 6, as per Objective-C.

gcc/objcp/ChangeLog:

	* Make-lang.in: Test in parallel with up to 6 instances.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
(cherry picked from commit f5d723ae07924307e5e9e6939112aed886f4aceb)