test_image_write.c: change from standalone to integrated with image_test.c

tests/test_image.c

@@ -1 +1,3 @@
+#define STB_IMAGE_STATIC
+#define STB_IMAGE_IMPLEMENTATION
 #include "stb_image.h"

tests/test_image_write.c

@@ -1 +1,3 @@
+#define STB_IMAGE_WRITE_IMPLEMENTATION
+#define STB_IMAGE_WRITE_STATIC
 #include "stb_image_write.h"