[TST] Added test_annotate in test_datetime.py

[mliu08]

PR summary

Added a smoke test for Axes.annotate in test.datetime under test_annotate to address the Axes.annotate task under #26864. These are the plots generated by the test code:

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PR checklist

• [ N/A] "closes #0000" is in the body of the PR description to link the related issue
• [N/A ] new and changed code is tested
• Plotting related features are demonstrated in an example
• [N/A ] New Features and API Changes are noted with a directive and release note
• [N/A ] Documentation complies with general and docstring guidelines

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