This makes a start on porting the Cogl conformance tests that currently
still live in the Clutter repository to be standalone Cogl tests that no
longer require a ClutterStage.

The main thing is that this commit brings in is the basic testing
infrastructure we need, so now we can port more and more tests
incrementally.

Since the test suite wants a way to synchronize X requests/replies and
we can't simply call XSynchronize in the test-utils code before we know
if we are really running on X this adds a check for an environment
variable named "COGL_X11_SYNC" in cogl-xlib-renderer.c and if it's set
it forces XSynchronize (dpy, TRUE) to be called.

By default the conformance tests are run off screen. This makes the
tests run much faster and they also don't interfere with other work you
may want to do by constantly stealing focus. CoglOnscreen framebuffers
obviously don't get tested this way so it's important that the tests
also get run on screen every once in a while, especially if changes are
being made to CoglFramebuffer related code.  On screen testing can be
enabled by setting COGL_TEST_ONSCREEN=1 in your environment.