Figure.meca: Refactor tests for plotting multiple focal mechanisms

[seisman]

Description of proposed changes

Followup PR of PR #2533.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[github-actions]

Summary of changed images

This is an auto-generated report of images that have changed on the DVC remote

Status	Path
deleted	pygmt/tests/baseline/test_meca_loc_array.png
deleted	pygmt/tests/baseline/test_meca_spec_2d_array.png
deleted	pygmt/tests/baseline/test_meca_spec_dataframe.png
renamed	pygmt/tests/baseline/test_meca_spec_dict_list.png -> pygmt/tests/baseline/test_meca_spec_multiple_focalmecha.png

Image diff(s)

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Path	Old	New

Report last updated at commit c17f091