Be compatible with PHPUnit installed with composer.
Be compatible with PHPUnit installed with composer.

Doing a class_exists() check is a simple way to check for PHPUnit being
installed via an autoloader. It also keeps things compatible with
a Vendor dir installation.

Fixes #3721

ionas/cakephp,dakota/cakephp,CakeDC/cakephp,justinyost/cakephp,timhoeppner/cakephp,ADmad/cakephp,CakeDC/cakephp,eryw/cakephp,eryw/cakephp,elboletaire/cakephp,erikfig/cakephp,tgr0ss/cakephp,burriko/cakephp,erikfig/cakephp,cakephp/cakephp,tisu2tisu/cakephp,davidyell/cakephp,CakeDC/cakephp,kumamidori/cakephp,Balachan27/cakephp,khoimt/cakephp-study,justinyost/cakephp,petteyg/cakephp,berrygoudswaard/cakephp,jeffblack360/cakephp,web-player/cakephp,dkarvounaris/cakephp,tenkoma/cakephp,berrygoudswaard/cakephp,bravo-kernel/cakephp,petteyg/cakephp,davidyell/cakephp,nrother/cakephp,cakephp/cakephp,bravo-kernel/cakephp,web-player/cakephp,pbaire/cakephp,eboominathan/cakephp,justinyost/cakephp,jerems02/cakephp,cakephp-fr/cakephp,HavokInspiration/cakephp,HavokInspiration/cakephp,et304383/passbolt_api,jerems02/cakephp,khoimt/cakephp-study,khoimt/cakephp-study,akintoey/cakephp,garethellis36/cakephp,burriko/cakephp,elboletaire/cakephp,akintoey/cakephp,dakota/cakephp,eryw/cakephp,web-player/cakephp,et304383/passbolt_api,ndm2/cakephp,tisu2tisu/cakephp,pbaire/cakephp,pbaire/cakephp,thinkingmedia/cakephp,yinhe007/cakephp,tenkoma/cakephp,dkarvounaris/cakephp,tisu2tisu/cakephp,HavokInspiration/cakephp,ndm2/cakephp,eboominathan/cakephp,cakephp-fr/cakephp,elboletaire/cakephp,gmponos/cakephp,nrother/cakephp,ADmad/cakephp,garethellis36/cakephp,jeffblack360/cakephp,petteyg/cakephp,cakephp/cakephp,CVO-Technologies/cakephp,kumamidori/cakephp,tenkoma/cakephp,jjj117/cakephp,jjj117/cakephp,dakota/cakephp,gmponos/cakephp,ndm2/cakephp,Balachan27/cakephp,dakota/cakephp,tgr0ss/cakephp,jeffblack360/cakephp,thinkingmedia/cakephp,nrother/cakephp,jerems02/cakephp,CakeDC/cakephp,Phillaf/cakephp,yinhe007/cakephp,ionas/cakephp,Phillaf/cakephp,garethellis36/cakephp,mvdriel/cakephp,bravo-kernel/cakephp,jjj117/cakephp,Phillaf/cakephp,ADmad/cakephp,gmponos/cakephp,CVO-Technologies/cakephp,akintoey/cakephp,cakephp/cakephp,garethellis36/cakephp,dkarvounaris/cakephp,mvdriel/cakephp,thinkingmedia/cakephp,davidyell/cakephp,michaelze/cakephp,ionas/cakephp,et304383/passbolt_api,timhoeppner/cakephp,berrygoudswaard/cakephp,adopico/cakephp,tgr0ss/cakephp,cakephp-fr/cakephp,CVO-Technologies/cakephp,adopico/cakephp,mvdriel/cakephp,Balachan27/cakephp,michaelze/cakephp,yinhe007/cakephp,et304383/passbolt_api,mvdriel/cakephp,erikfig/cakephp,tenkoma/cakephp,timhoeppner/cakephp,petteyg/cakephp,ADmad/cakephp,ndm2/cakephp,et304383/passbolt_api,michaelze/cakephp,burriko/cakephp,eboominathan/cakephp,adopico/cakephp,kumamidori/cakephp