G-API: Add output allocation tests for backends (#15012)

* Add output tests for backends

* Fix large size test: output is in fact reallocated

* Use cv::Mat copies for reallocation tracking

* Separate LargeSizeWithCorrectSubmatrix test

* Rename backed output allocation tests

* Address code review feedback

Update test names

Add illustrative "expect (non-)empty" checks

Rename mat "copy" to mat reference

Add more pointer checks

* Add illustrative checks