Update PhpUnitWrapper.php & TestRunner.php to recommend PHPUnit installation via Composer rather than PEAR

Using the PEAR Installer to install PHPUnit is no longer supported.  See https://github.com/sebastianbergmann/phpunit/wiki/End-of-Life-for-PEAR-Installation-Method.

Replace PEAR in a comment and die output message with Composer.

Signed-off-by: AntonyThorpe <antony.thorpe@budget12.com>
Merge pull request #3476 from AntonyThorpe/PEAR-PHPUnit

Update PhpUnitWrapper.php & TestRunner.php to recommend PHPUnit installation via Composer