tests/fw/graphics/util: fix platform suffix for Linux CI

Restore platform suffix on Linux (e.g. ~spalding) while keeping the
additional -darwin suffix for macOS local development. This matches
the naming convention of the PNG fixture files in the repository.

Co-authored-by: Claude <claude@anthropic.com>

Signed-off-by: Joseph Mearman <joseph@mearman.co.uk>

Author: Mearman

tests/fw/graphics/util.h

@@ -58,11 +58,11 @@ static const char *namecat(const char* str1, const char* str2){
     printf("filename and filename_xbit %s : %s\n", filename, filename_xbit);
   } else {
 #if !PLATFORM_DEFAULT
-    // On macOS, append ~platform-darwin suffix to allow different fixtures for local dev
-    // Linux (CI) uses the base fixture name without any platform suffix
-#if defined(__APPLE__)
+    // Append platform suffix for non-default platforms
     strcat(filename, "~");
     strcat(filename, PLATFORM_NAME);
+#if defined(__APPLE__)
+    // On macOS, also append -darwin to differentiate local dev fixtures from CI
     strcat(filename, "-darwin");
 #endif
 #endif