Use xarray.testing rather than pygmt.grdinfo in grdlandmask tests

[willschlitzer]

Description of proposed changes

This PR refactors the pygmt.grdlandmask tests to use xarray.testing.assert_allclose() rather than pygmt.grdinfo() in preparation for the deprecation associated with better return values for grdinfo (#593).

Fixes #

Reminders

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• 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.

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