Add code coverage

[atifaziz]

Several recent refactorings have highlighted that there isn't sufficient code coverage to refactor confidently. A case in point is issue #649 that seems to be mainly caused by me.

Coverlet has served me well in a lot of projects as a coverage tool. We could consider using it here too.

[Orace]

I quote myself from #667

In unit testing pattern we should not even use TestingSequence since it tests multiple things. (Enumerator disposal, multiple enumeration, multiple MoveNext at the end of the source).
The proper (in terms of unit testing) way to work is to split TestingSequence in 3 new mock (TestingSequenceForDisposed, etc..), add one test that use each of it and finally add revelant cases.

Indeed, an exception for a call to MoveNext after a sequence end may hide a "sequence not disposed" issue.

And if a method take say N sequence in input, we should multiply the tests and use the testing sequence only once in each test.

Indeed an exception from one input sequence may hide a problems with an other input sequence. In such a case, tests multiplication allow to have two separate red tests.

Should we go for orthodox pure unit testing?