build: test-driver.scm: Enable colored test results by default.

The Automake parallel test harness does its own smart detection of the
terminal color capability and always provides the --color-tests argument to
the driver.  This change defaults the --color-tests argument to true when the
test driver is run on its own (not via Automake).

* build-aux/test-driver.scm (main): Set the default value of the --color-tests
argument to true when it's not explicitly provided.