Update testing/index.md for clearer PHPUnit installation instructions

Step by step Composer installation instructions, including a composer.json example.
In addition:
- Removed self-page reference to introduction
- Noted End of Life for PEAR Installation Method
- Removed reference to Ruby as this doesn't add value
- Shortened testing via Web Browser section (as covered in Composer installation instructions)
I would say we just remove that as a documented option, not much point keeping it given how heavily we push for composer in other places.