Add mock.ElementsMatch argument matcher

[brackendawson]

Summary

Add an argument marcher to mock which matches if array/slice elements match.

Changes

• Add mock.ElementsMatch() argument matcher.
• Refactor mock.argumentMatcher to be an interface, it pretty much already was.
• Move common code used by both assert and mock into an internal package.

Motivation

There is already mock.MatchedBy which allows users to write this themselves, but it's not trivial and a lot of people seem to need this specific matcher.

func TestIfy(t *testing.T) {
	m := &myMock{}
	m.On("Do", mock.ElementsMatch([]int{1, 2, 3})).Return()
	m.Do([]int{2, 3, 1})
}

Related issues

Closes #1117
Closes #1180

[dolmen]

This MR includes changes to use internal/check. They are unrelated to the description of this issue. Please makes those changes available in a separate MR.

[brackendawson]

@dolmen done in #1431, this will need rebasing somewhat after that is merged.

[dolmen]

We must first cleanup the argument matching API. See my proposal in #1578.

It is also too early to merge #1431: we have plenty of pending patches in the queue that would be broken by #1431, so I would like that we review everything before such a major refactor.

In light of this, I think that this implementation is not useful at this point, so I'm closing the issue. Could you instead open an issue with an API proposal that would fit #1578?