Migrate Figure.subplot tests to dvc

[michaelgrund]

Description of proposed changes

Related to #1131.

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 adding new functionality, add an example to docstrings or tutorials.

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
added	pygmt/tests/baseline/test_subplot_autolabel_margins_title.png
added	pygmt/tests/baseline/test_subplot_basic_frame.png
added	pygmt/tests/baseline/test_subplot_clearance_and_shared_xy_axis_layout.png
added	pygmt/tests/baseline/test_subplot_direct.png

Image diff(s)

• pygmt/tests/baseline/test_subplot_autolabel_margins_title.png

• pygmt/tests/baseline/test_subplot_basic_frame.png

• pygmt/tests/baseline/test_subplot_clearance_and_shared_xy_axis_layout.png

• pygmt/tests/baseline/test_subplot_direct.png

Report last updated at commit 7513b6c

[weiji14]

Btw, we should teach you about making batch changes to save on a few clicks. See https://docs.github.com/en/github/collaborating-with-issues-and-pull-requests/incorporating-feedback-in-your-pull-request#applying-suggested-changes. You can go to the 'Files' tab on the GitHub UI and click on 'Add suggestion to batch'.

[michaelgrund]

Btw, we should teach you about making batch changes to save on a few clicks. See https://docs.github.com/en/github/collaborating-with-issues-and-pull-requests/incorporating-feedback-in-your-pull-request#applying-suggested-changes. You can go to the 'Files' tab on the GitHub UI and click on 'Add suggestion to batch'.

Thanks for mentioning @weiji14, didn't know that so far 😄.

[weiji14]

Looks good @michaelgrund, feel free to merge this in yourself. I've disabled the 'Docs' CI test requirement (since they don't run on changes to `pygmt/tests/*') so you should be able to merge it in straight this time.