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)
You could also describe adding it as a simple one line command, this would make the docs much easier to follow:

`composer require "phpunit/phpunit:*@stable"`