Allow Plugin::getCollection() to be used
Allow Plugin::getCollection() to be used

Add this method to the public API of `Plugin` as it has some use cases
for plugin test case setup.

Refs #12682

ndm2/cakephp,ndm2/cakephp,cakephp/cakephp,garethellis36/cakephp,mvdriel/cakephp,CakeDC/cakephp,cakephp/cakephp,tenkoma/cakephp,mvdriel/cakephp,ndm2/cakephp,petteyg/cakephp,cakephp/cakephp,ADmad/cakephp,mvdriel/cakephp,garethellis36/cakephp,garethellis36/cakephp,petteyg/cakephp,ADmad/cakephp,CakeDC/cakephp,ADmad/cakephp,dakota/cakephp,CakeDC/cakephp,tenkoma/cakephp,petteyg/cakephp,ndm2/cakephp,dakota/cakephp,tenkoma/cakephp,petteyg/cakephp,cakephp/cakephp,mvdriel/cakephp,dakota/cakephp,CakeDC/cakephp,garethellis36/cakephp,ADmad/cakephp,dakota/cakephp,tenkoma/cakephp