Use automake's support for parallel test execution.

On my system this speeds up 'make -j8 check' by about
50%. The drawback is that we don't get to see Greg's
nice output from the test programmes. It's hidden in
a log file, even on failure or 'make V=1'. Probably
this is better for most people; developers can run
the tests manually.

Makefile.am

-AUTOMAKE_OPTIONS = subdir-objects
+AUTOMAKE_OPTIONS = subdir-objects color-tests parallel-tests
 
 lib_LTLIBRARIES = libopus.la