Allow running the test suite without the pytest-mpl plugin

[ericpre]

The pytest-mpl plugin is used to compare the plot output with reference images and if pytest-mpl is not installed, the test suite fails. Sometimes, it is convenient to run the test without pytest-mpl installed and this PR fixes it.

Progress of the PR

• Fix test suite failure when pytest-mpl is not installed.
• ready for review.

[codecov]

Codecov Report

Merging #2624 (1b0c27a) into RELEASE_next_patch (922cbc9) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@                  Coverage Diff                   @@
##           RELEASE_next_patch    #2624      +/-   ##
======================================================
- Coverage               76.58%   76.58%   -0.01%     
======================================================
  Files                     202      201       -1     
  Lines                   29698    29673      -25     
  Branches                 6497     6496       -1     
======================================================
- Hits                    22745    22724      -21     
+ Misses                   5187     5183       -4     
  Partials                 1766     1766              

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[jlaehne]

LGTM