tests/functional/test_aarch64_virt: Fix vulkan test without egl-headless

tests/functional/test_aarch64_virt: Fix vulkan test without egl-headless

The vulkan test currently fails if the egl-headless device is not
available. Let's add a proper check to skip the test in this case.

Fixes: 3d30f882ce ("tests/functional: extend test_aarch64_virt with vulkan test")
Reported-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Cornelia Huck <cohuck@redhat.com>
Message-ID: <20250205141140.97437-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>