fix: align tests with the new help and messages

The --skip-packs help and the validate_pr fetch error were reworded, and a batocera page with no tab bar is plain Markdown that keeps the underscore entity, read here as a browser shows it.

scripts/validate_pr.py

@@ -632,7 +632,7 @@ def _project_main_line() -> str | None:
     if fetched.returncode != 0:
         raise RuntimeError(
             f"this clone does not hold the main of {PROJECT_URLS[0]}, and "
-            f"fetching it from {source} failed: {fetched.stderr.strip()}"
+            f"fetching its main from {source} failed: {fetched.stderr.strip()}"
         )
     return "FETCH_HEAD"