sdk-vb-fullscreen: Use MiniBrowser and WebGL to test rendering

Since the rebase to Buster we are no longer shipping `clutter/test-text`,
making the test execution impossible to complete.

Replace the test program with the MiniBrowser from WebKitGTK opening
a simple WebGL scene and testing that the output is not stretched.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>