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)
Thanks, have followed your good advice in the latest commit, however, have used:
composer require --dev "phpunit/phpunit:3.7.*@stable"
PHPUnit 3.7.37 works in my browser.  Please advise.