tests/igt_command_line.sh: Add new selftest for special handling

dmabuf_selftests has been recently added to the test list but not added
to the special handling in igt_command_line.sh and therefore build
testing would fail.

Signed-off-by: Antonio Argenziano <antonio.argenziano@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
[Fixed syntax error and test name]
Reviewed-by: Petri Latvala <petri.latvala@intel.com>