Make the TestCaseCovers sniff to detect multiple @coversDefaultClass cases

[stronk7]

Recently I was playing with code-coverage generation and when running PHPUnit it was detected that we have a few unit tests having multiple @coversDefaultClass annotations.

That's plain wrong, only one is allowed (in fact PHPUnit stops when that happens). Such an annotation is about to define the default class to apply when the @covers annotations do no specify a class. Just that.

So we should start erroring in the Sniff when multiple are detected. Of course, only that annotation, classes and methods are allowed to have multiple @covers ones.

Ciao :-)

[stronk7]

Cross-linking with MDL-82148 that is the one going to fix current cases.