Improve `TestingSequence` assertions

[viceroypenguin]

This PR updates the assertions made in TestingSequence:

• Per the specification, it is not a failure to call .Dispose() multiple times. The .Dispose() method is expected to be idempotent, such that it is callable multiple times without throwing an exception. As such, we should not be afraid to take advantage of such behavior when it makes code easier.
• Per the specification, it is not a failure to call .MoveNext() after receiving a false response. The enumerator is simply expected to continue to return false for each following call. As such, we should not be afraid to take advantage of such behavior when it makes code easier (see #905 for examples).
• While most IEnumerators do not complain when calling .MoveNext() after disposal, it does indicate an error in our code to expect that .MoveNext() is a valid behavior after we have disposed the iterator. As such, we should fail directly.
• While most IEnumerators return a default or the last value when calling .Current after .MoveNext() returns false, the spec does not make any promises on the usefulness of .Current in this situation. More importantly, we should be relying on .MoveNext() return value and not attempting to reference .Current in these cases. As such, we should fail directly.
• While most IEnumerators do not complain when calling .Current after disposal, it does indicate an error in our code to expect that .Current is a valid behavior after we have disposed the iterator. As such, we should fail directly.

[codecov[bot]]

Codecov Report

Merging #901 (74faa1f) into master (c081197) will not change coverage. The diff coverage is n/a.

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  Coverage   92.41%   92.41%           
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  Files         112      112           
  Lines        3439     3439           
  Branches     1021     1021           
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  Hits         3178     3178           
  Misses        199      199           
  Partials       62       62           

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[viceroypenguin]

A future PR will make additional improvements, similar to viceroypenguin/SuperLinq#143.

• Expressing exactly how many enumerations are allowed
• Unit testing TestingSequence to prove it tests behavior correctly.

[atifaziz]

This has been superseded by PR #936.