tests: derive XglDevice from xgl_testing::Device

as well as removing unused members.

v2: call xgl_testing::set_error_callback()
diff --git a/tests/init.cpp b/tests/init.cpp
index 0d16e5b..8177c21 100644
--- a/tests/init.cpp
+++ b/tests/init.cpp
@@ -747,5 +747,6 @@
 
 int main(int argc, char **argv) {
     ::testing::InitGoogleTest(&argc, argv);
+    xgl_testing::set_error_callback(test_error_callback);
     return RUN_ALL_TESTS();
 }