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)
Latest commit removes web browser reference and changes the Composer command to: composer require --dev "phpunit/phpunit:*@stable", as previously advised.  Made a note that 3.7.*@stable is needed for runnings tests via the browser.