Build sklearn pipeline tests for visualizers

[pdamodaran]

This PR fixes #1257 (If you are fixing a bug) which reported a bug that caused a problem to occur when users...

(or if you are introducing a new feature) which requested a feature to allow the user to...

I have made the following changes:

1. Added pipeline tests for the following:
   • DiscriminationThreshold
   • LearningCurve
   • FeatureImportances
   • ResidualsPlot
   • AlphaSelection

Sample Code and Plot

If you are adding or modifying a visualizer, PLEASE include a sample plot here along with the code you used to generate it.

TODOs and questions

Still to do:

• [Add a pipeline model input test for feature importances (if possible)]
• [Add a pipeline model input test for alpha selection (if possible)]

CHECKLIST

• [x] Is the commit message formatted correctly?
• [x] Have you noted the new functionality/bugfix in the release notes of the next release?

• [x] Have you added/updated unit tests where appropriate?
• [x] Have you updated the baseline images if necessary?
• [x] Have you run the unit tests using pytest?
• [x] Is your code style correct (are you using PEP8, pyflakes)?

[codecov[bot]]

Codecov Report

Merging #1259 (2eb88c9) into develop (cc7a415) will increase coverage by 0.07%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #1259      +/-   ##
===========================================
+ Coverage    90.81%   90.88%   +0.07%     
===========================================
  Files           93       93              
  Lines         5301     5301              
===========================================
+ Hits          4814     4818       +4     
+ Misses         487      483       -4     

Impacted Files	Coverage Δ
yellowbrick/classifier/threshold.py	99.35% <0.00%> (+0.64%)	:arrow_up:
yellowbrick/regressor/residuals.py	90.56% <0.00%> (+0.94%)	:arrow_up:
yellowbrick/regressor/alphas.py	95.60% <0.00%> (+1.09%)	:arrow_up:
yellowbrick/model_selection/learning_curve.py	100.00% <0.00%> (+1.88%)	:arrow_up:

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[pdamodaran]

Thanks @lwgray!