Add UsingTestingSequence to enforce simpler & correct usage

[atifaziz]

This PR obsoletes the use of AsTestingSequence in favour of the new UsingTestingSequence. This is done to enforces simpler & more correct usage.

TestingSequence is primarily designed to help test two things in an operator:

1. It doesn't enumerate the sequence more than once
2. It disposes the sequence

However, most uses seemed to be exercising the first but not the second. The latter was probably often skipped/missed because it required more non-obvious setup; an explicit using block (non-obvious usage) as well as eager evaluation before assertion of results. Take the following example:

var result = sequence.AsTestingSequence().Rank();

This will only ensure that Rank doesn't iterate sequence more than once when it's iterated during testing. To also test that it disposes the source iterator when done, the above has to be written as:

IEnumerablt<int> result;
using (var ts = sequence.AsTestingSequence())
    result = ts.Rank().ToArray();

With UsingTestingSequence, you just do:

var result = sequence.UsingTestingSequence(ts => ts.Rank());

TestingSequence Uses Needing Review

• CompareCountTest
• CountDownTest
• EndsWithTest
• EquiZipTest
• FlattenTest
• FoldTest
• InterleaveTest
• MoveTest
• NestedLoopTest
• RankTest
• SortedMergeTest
• StartsWithTest
• TakeLastTest
• TestingSequence
• TransposeTest
• ZipLongestTest
• ZipShortestTest

[leandromoh]

LGTM

[leandromoh]

I think all the explanation and operator list should be in a issue. So we can create PR for each operator.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.39%. Comparing base (1fc1e31) to head (b39e8d7).
Report is 132 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #475   +/-   ##
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  Coverage   92.39%   92.39%           
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  Files         112      112           
  Lines        3447     3447           
  Branches     1021     1021           
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  Hits         3185     3185           
  Misses        200      200           
  Partials       62       62           

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