Added seeFormErrorMessages function

[TavoNiievez]

It is also present in the Laravel module.

A validation was added to allow omitting => null in the array if you don't want to specify the error message.

[DavertMik]

Thanks @TavoNiievez for PR
Thanks @Naktibalda for a review

My biggest concern here that this method validates full error messages, which is useless in most of cases. Those error messages are either standard (so no reason to test the full message) or are subject to change in time.

Consider the case - we try to improve error messages, and we change them on each iteration adding more and more descriptinve erros and more and more polite languages, while keeping validation rules the same. Should we update test on each wording change? I prefer not. So personally I would assert for the error field to include some text.

However, not sure how to do this correctly with consistent API

[TavoNiievez]

@DavertMik if i understand you correctly, what you are asking is that an assertStringContainsString be made to be able to pass only a fragment of the error message instead of the whole message.

Edit: That is the current behavior of seeFormErrorMessage, more documentation was added explaining that you can include a part of the error message as well.

[TavoNiievez]

@Naktibalda @DavertMik I've made all the requested changes based on your feedback,

thanks to both of you.