oeqa/selftest: test_testimage_virgl_gtk_sdl: run only if gtk or sdl are enabled in...

oeqa/selftest: test_testimage_virgl_gtk_sdl: run only if gtk or sdl are enabled in base config

The test was forcibly enabling both gtk and sdl in qemu and then running the test.

This changes the logic to skip the test if neither is enabled,
and run gtk and/or sdl tests only if either is already enabled
in the base config that was in place before running the selftest.

Together with disabling sdl support in qemu by default this effectively
skips the test on the yocto autobuilder, resolving the selftest
failures seen with qemu 11.0.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>