fix tests

[mcocdawc]

Description

Add the mpl references for tests and remove the directory from the .gitignore.

The only remaining failing tests are:

FAILED tests/images/test_healpix.py::test_healpix_interpolated - ImportError: Regridding not available. Please install the earthkit-regrid package.
FAILED tests/images/test_healpix.py::test_healpix_pixels - ImportError: The healpix plotting backend requires the healpy package.
FAILED tests/images/test_octahedral.py::test_octahedral_interpolated - ImportError: Regridding not available. Please install the earthkit-regrid package.

As the earthkit-regrid package is deprecated AFAIK, I don't know if the 1st and 3rd test should be fixed or simply removed.
Re the 2nd test. Either healpy should be added as a strict dependency, or the test should only be executed if healpy is installed. That's a call one of the maintainers has to make.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

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[mcocdawc]

now included in #176