Fix the test suite with Automake < 1.13

The test suite uses a LOG_COMPILER, which requires the parallel-tests
Automake test driver which is the default only since version 1.13, so
explicitly enable it so it is used by earlier versions too.

Moreover, the parallel-tests test diver was introduced by Automake
1.11, as was the option, so we need to depend on it too.  It was
released 2009-05-17, which is 2 months after GTK 2.16, the minimum GTK
version we depend on (2009-03-13).  If this is a problem, we could
also rework the Automake test layout to use the older and a little
deprecated serial-tests test driver that is the historical one.