Create tests for `is_fitted` parameter for ResidualsPlot and PredictionError

[lwgray]

This PR adds tests to #1221. These tests assure us that the is_fitted param's state is maintained when the visualizer is instantiated.

I have made the following changes:

1. Added test for ResidualPlots
2. Added test for PredictionError
3. Removed unnecessary import so that flake8 passed

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

• [ ] Included a sample plot to visually illustrate your changes?
• [x] Do all of your functions and methods have docstrings?
• [x] Have you added/updated unit tests where appropriate?
• [ ] 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)?
• [ ] Have you documented your new feature/functionality in the docs?

• [ ] Have you built the docs using make html?

[codecov[bot]]

Codecov Report

Merging #1223 (8828449) into develop (4be320d) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1223   +/-   ##
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  Coverage    90.45%   90.45%           
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  Files           91       91           
  Lines         5135     5135           
========================================
  Hits          4645     4645           
  Misses         490      490           

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