`skip` header, better output & output folder checks

• CR #8:
  • added skip header in cmd.p1 to skip individual tests
  • run tests in lexical order
• CR #7
  • on exit code show stdout / stderr cleanly ().
  • using diffy to show diff when stdout/stderr don't match

Before:

After:

• CR #9: checking if output folder content matches (in case of failures the output we print is buggy in case of missing/extra files).