test: Add some common code to load test resources

As well as reducing duplication, this lets the tests load their resources
from the SDL_GetBasePath() on platforms that support it, which is useful
if the tests are compiled along with the rest of SDL and installed below
/usr as manual tests, similar to GNOME's installed-tests convention.

Thanks to Ozkan Sezer for the OS/2 build glue.

Co-authored-by: Ozkan Sezer <sezeroz@gmail.com>
Signed-off-by: Simon McVittie <smcv@collabora.com>

test/testutils.h

@@ -0,0 +1,17 @@
+/*
+Copyright 1997-2022 Sam Lantinga
+Copyright 2022 Collabora Ltd.
+SPDX-License-Identifier: Zlib
+*/
+
+#ifndef TESTUTILS_H
+#define TESTUTILS_H
+
+#include "SDL.h"
+
+SDL_Texture *LoadTexture(SDL_Renderer *renderer, const char *file, SDL_bool transparent,
+                         int *width_out, int *height_out);
+char *GetNearbyFilename(const char *file);
+char *GetResourceFilename(const char *user_specified, const char *def);
+
+#endif