Add/upgrade string indexing tests

[crupley]

Related issues Improve test coverage for transformers & estimators https://github.com/salesforce/TransmogrifAI/issues/278

Describe the proposed solution

• Separate tests for OpStringIndexer and OpStringIndexerNoFilter
• Upgrade tests to use Op Transformer/Estimator Spec
  • OpStringIndexerNoFilterTest
  • OpIndexToStringNoFilterTest

Describe alternatives you've considered

Attempted to upgrade OpStringIndexerTest and OpIndexToStringTest but these both extend an wrapper-type class that is not compatible with the new test spec.

Additional context None

[tovbinm]

@crupley can you elaborate what's not working with the base specs and wrapper stages? (cc @snabar)

[crupley]

@tovbinm, The signature of the transformer spec is:

abstract class OpTransformerSpec[O <: FeatureType : WeakTypeTag : ClassTag,
TransformerType <: OpPipelineStage[O] with Transformer with OpTransformer : ClassTag]

The transformer passed must inherit from OpPipelineStage, Transformer, and OpTransformer. OpTransformerWrapper is missing the OpTransformer piece.

[codecov[bot]]

Codecov Report

Merging #294 into master will increase coverage by <.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #294      +/-   ##
==========================================
+ Coverage    86.4%   86.41%   +<.01%     
==========================================
  Files         319      319              
  Lines       10452    10452              
  Branches      346      346              
==========================================
+ Hits         9031     9032       +1     
+ Misses       1421     1420       -1

Impacted Files	Coverage Δ
...ce/op/stages/impl/feature/OpOneHotVectorizer.scala	96.77% <ø> (ø)	:arrow_up:
...scala/com/salesforce/op/test/OpEstimatorSpec.scala	93.33% <ø> (ø)	:arrow_up:
...p/stages/impl/feature/TextMapPivotVectorizer.scala	100% <ø> (ø)	:arrow_up:
...ages/impl/feature/MultiPickListMapVectorizer.scala	100% <ø> (ø)	:arrow_up:
...force/op/stages/impl/feature/OpIndexToString.scala	100% <0%> (+50%)	:arrow_up:

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[tovbinm]

Ha! True. Its actually on purpose :) ok! So we need a new base class for testing wrapped stages then.

[tovbinm]

@crupley here you go - https://github.com/salesforce/TransmogrifAI/pull/295