Update 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
- Noted End of Life for PEAR Installation Method
- Removed reference to Ruby as doesn't add value
- Shortened testing via Web Browser section (as covered in Composer installation instructions)

Signed-off-by: AntonyThorpe <antony.thorpe@budget12.com>
Reworded sentence to remove ambiguity regarding PHPUnit versions

Signed-off-by: AntonyThorpe <antony.thorpe@budget12.com>
Changed title from "An Option" to "Symlinking the PHPUnit Binary"

Signed-off-by: AntonyThorpe <antony.thorpe@budget12.com>
Removed reference to PEAR in PHPUnit installation instructions

Removed Via PEAR section
Reworded title

Signed-off-by: AntonyThorpe <antony.thorpe@budget12.com>