test: allow skipping tests on Xephyr with GLES

The GLES tests need an actual GPU (/dev/dri/* device), which is not available
within github CI runners, so we need to skip those when running there.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

test/meson.build

@@ -112,7 +112,11 @@ if get_option('xvfb')
     endif
 
     if get_option('xephyr') and build_glamor
-        foreach testsuite : ['','-gles2','-gles3']
+        glamor_tests = ['']
+        if get_option('test_xephyr_gles')
+            glamor_tests += ['-gles2','-gles3']
+        endif
+        foreach testsuite : glamor_tests
             test_env = piglit_env
             if(testsuite == '-gles2')
                 test_env = gles20_env