Ignore test-driver

The parallel tests harness from Automake, enabled by default [since automake 1.12b](http://git.savannah.gnu.org/cgit/automake.git/commit/?id=5e771b2) and in previous versions by the `parallel-tests` option in `AM_INIT_AUTOMAKE`, uses the above script to log the tests output if no other driver is specified with the [`($prefix_)LOG_DRIVER` variables](http://git.savannah.gnu.org/cgit/automake.git/tree/bin/automake.in#n4580).