Fix pygmt/helpers/testing.py doctest for GMT 6.2.0rc1

[weiji14]

Description of proposed changes

Update doctests in pygmt/helpers/testing.py's check_figures_equal function failing because of new GMT 6.2.0 themes default. Specifically caused by the change in MAP_FRAME_AXES from WESNZ to auto (WrStZ for Cartesian figures).

Address #1217

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