Allow easy XDEBUG debugging with autotest

Debugging scripts needs the environment variable XDEBUG_CONFIG to be
set, which initiates a connection to a listening debugger. But if that
env is set and one of the PHP setup steps occurs (e.g. occ install), the
debugging session is 'completed' and the debugger no longer listens
during the actual tests. This saves the XDEBUG_CONFIG env so it only
applies during the PHPUnit execution, so only the real tests trigger the
debugging.