tests: Use full include path for gtest

For some reason, QtCreator doesn't like the include "gtest/gtest.h"
it just won't do symbol lookup.
diff --git a/tests/test_common.h b/tests/test_common.h
index fd9e235..f93ae08 100644
--- a/tests/test_common.h
+++ b/tests/test_common.h
@@ -10,6 +10,7 @@
 #include <xgl.h>
 
 #include "gtest/gtest.h"
+#include "gtest-1.7.0/include/gtest/gtest.h"
 
 #define ASSERT_XGL_SUCCESS(err) ASSERT_EQ(XGL_SUCCESS, err) << xgl_result_string(err)