Refactor `StormLaws`

[ttim]

In this PR I refactored StormLaws class with a way to test same Producer on both Storm and Memory platforms and compare their results. In future this approach can be extended to Scalding platform as well with a very unified way of testing.

Also I've included test for https://github.com/twitter/summingbird/issues/725, where we build Storm topology which fails with ClassCastException in runtime.

[codecov-io]

Codecov Report

Merging #727 into develop will decrease coverage by 0.35%. The diff coverage is 94.82%.

@@             Coverage Diff             @@
##           develop     #727      +/-   ##
===========================================
- Coverage    71.38%   71.02%   -0.36%     
===========================================
  Files          141      144       +3     
  Lines         3491     3548      +57     
  Branches       195      194       -1     
===========================================
+ Hits          2492     2520      +28     
- Misses         999     1028      +29

Impacted Files	Coverage Δ
...a/com/twitter/summingbird/MemoryTestExecutor.scala	100% <100%> (ø)
...n/scala/com/twitter/summingbird/TestPlatform.scala	100% <100%> (ø)
...cala/com/twitter/summingbird/storm/TestStore.scala	70.58% <100%> (ø)	:arrow_up:
.../com/twitter/summingbird/PlatformTransformer.scala	90.32% <90.32%> (ø)
...a/com/twitter/summingbird/storm/StormTestRun.scala	60% <0%> (-30%)	:arrow_down:
...la/com/twitter/summingbird/storm/BuildSummer.scala	42.22% <0%> (-22.23%)	:arrow_down:
...ain/scala/com/twitter/summingbird/TestGraphs.scala	88.53% <0%> (-5.1%)	:arrow_down:
... and 1 more

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[ttim]

@johnynek @pankajroark thoughts on this one?

[johnynek]

👍

Nice cleanup.

[ttim]

@johnynek What I really like here is a fact we can introduce shared test suite for Storm and Scalding platforms after introduction of proper memory platform (with notion of time) and Scalding implementation for CreatorCtx.

Should I merge or better to wait for one more review from @pankajroark for example?

[ttim]

@johnynek @pankajroark Hi, I've reworked this change a lot with introduction of TestPlatform which corresponds to previous CreatorCtx, can you take a look? I think it's much more Summingbirdish approach.