Add TestLogger::setCollectContext and fluent interface

Add TestLogger::setCollectContext and fluent interface

TestLogger has a setCollect() but lacked a similar method for the
$collectContext introduced with:
    7b85b6b09e - Optionally collect context in TestLogger

Document $collectContext on __construct()
Add setCollectContext()
Make both setCollect() and setCollectContext() to return $this for
fluent interface:

  $logger = new \TestLogger();
  $logger
      ->setCollect( true ),
      ->setCollectContext( true );

Change-Id: I806b07cfddbc7b428d41e3d619ef4d1db16f6747