Drop test suite screenshooter protocol extension

Until now we had two different protocol extensions: one for the test suite screenshooter and other for the screenshooter client. As they are identical and this won't change, drop the extension being used by the test suite and make it use the same as the screenshooter client.

This allow us to remove the test suite screenshooter implementation in favor of Weston's default screenshooter.

Besides the cleanup, it also allow us to use the DRM writeback screenshooter in the test suite, as the test suite implementation was hardcoded to use a renderer based screenshooter.