Test suite: Run weston/wayland under weston/headless
Modify the test suite to run weston/wayland under weston/headless for a second run of the screenshot tests. The nested weston will have Mesa fall back to software GL, so we get the GL-renderer tested. It should also work in all CI runners without requiring drivers like hw GL does.
This depends on #121 (closed) because modifying the autotools build is painful.
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