Add quiet / failures-only option

[Chemaclass]

📚 Description

Display Only Failing Tests (Quiet Mode) // Idea from @akinomyoga

Goal: Suppress passed tests and show only failures to reduce output noise.

Steps:

• Introduce a CLI flag: --failures-only or --quiet.
• When enabled:
  • Suppress all "✓ Passed" lines.
  • Show only failures or summary.
• Optional: Show a final count of passed/failed/skipped.

Example:

bashunit --failures-only

🔖 Changes

• Added new CLI flag -q|--failures-only|--quiet to enable quiet mode
• Documented usage of bashunit --failures-only and its effect in the command-line guide
• Updated help output to show the quiet mode option
• Documented BASHUNIT_FAILURES_ONLY environment variable in the configuration guide with an example
• Implemented quiet mode logic via new env helpers, early return in state::print_line, and display in verbose output

✅ To-do list

• [x] I updated the CHANGELOG.md to reflect the new feature or fix
• [x] I updated the documentation to reflect the changes