Currently all OpenGL and OpenCL tests are generated regardless of
whether a user has configured piglit to test that API. That means we
waste cycles and disk space on tests that are probably not interesting.

This patch changes that by creating two separate targets, one for OpenCL
tests and one for OpenGL tests (these targets are always available, so
developers working on generators can still run ninja gen-{gl,cl}-tests
to generate those tests even if they didn't configure them), but are
conditionally added to the meta-target gen-tests.

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>