[test] Use shared headless display

Prevents some OpenGL implementations from bailing out.

test/api/query.test.cpp

@@ -26,7 +26,7 @@ public:
     }
 
     util::RunLoop loop;
-    HeadlessBackend backend;
+    HeadlessBackend backend { test::sharedDisplay() };
     OffscreenView view { backend.getContext() };
     StubFileSource fileSource;
     ThreadPool threadPool { 4 };