Lag : Add test

[Orace]

Fix a part of #649
Fork of #660

[atifaziz]

I am struggling to understand the value proposition of this test. If it's to make sure that TestingSequence is used then why not add its use to the existing tests instead of isolating it to one test?

[Orace]

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).

I'm agree that TestLagPassesCorrectLagValuesOffsetBy1 and TestLagPassesCorrectLagValuesOffsetBy2 can be replaced by new cases in TestLagOnKnownInput

The proper (in terms of unit testing) way to work is to split TestingSequence in 3 new mock (TestingSequenceForDisposed, etc..), add one test for each of it, add relevant cases for each of it.

I don't really want to push that since it's a lot of rework..