Silence PytestUnknownMarkWarning by setting marker for pytest-benchmark

[weiji14]

Description of proposed changes

Registering custom marker for pytest-benchmark in pyproject.toml to silence PytestUnknownMarkWarning.

Alternative solution would be to install pytest-benchmark in all the Continuous Integration workflow files.

References:

• https://docs.pytest.org/en/7.4.x/example/markers.html#registering-markers
• https://stackoverflow.com/questions/60806473/pytestunknownmarkwarning-unknown-pytest-mark-xxx-is-this-a-typo

Patches #2908 (comment)

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

[weiji14]

No more warnings in https://github.com/GenericMappingTools/pygmt/actions/runs/7320250546/job/19939104443?pr=2912#step:8:723.

[weiji14]

For reference: text is copied from https://github.com/ionelmc/pytest-benchmark/blob/cec69f5fcf793321fb1ec5c961bd2c6a3953a3c0/src/pytest_benchmark/plugin.py#L482