tests: Allow tests to use customized command line parameters

Tests will now return the extra command line parameters they need
when run with --params
tests: Add record_screenshot

This adds a record_screenshot request to the test module as well as
the supporting infrastructure in compositor.c/h
compositor-headless: Add screenshot capabilities

NOTE: This commit is a placeholder,  the final code won't use ppm files :)
tests: add test of headless compositor screenshot mechanism

NOTE: this currently dumps a ppm into the weston source dir - not ideal...