Updated en/topics/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 an introduction
- Removed reference to PEAR installation instructions due to end of life
- Removed reference to Ruby as doesn't add value
- Shortened testing via Web Browser section (as covered in Composer installation instructions)
Describe adding PHPUnit as a simple one line command

- Removed the composer.json example
- Removed comment about PHPUnit versions

Signed-off-by: AntonyThorpe <antony.thorpe@budget12.com>
Updated for PHPUnit version clarification

Updated Composer command to reference latest stable version as advised
Removed testing installation via Browser step
Added a note about the PHPUnit 3.7 requirement for running tests via the Browser
Merge pull request #3475 from AntonyThorpe/PHPUnitDocs

Update testing/index.md for clearer PHPUnit installation instructions