Fix the failing tests for the updated gravimetric grids

[weiji14]

Description of proposed changes

Specifically the earth_faa, earth_faaerror, earth_ndefl, earth_edefl and earth_vgg grids. Xref GenericMappingTools/gmtserver-admin#291

Fixes #

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[Esteban82]

Out of curiosity: what is the main goal of these tests?
Are they meant to verify that the provided grids are correct, to check that the command runs as expected, or something else?

[weiji14]

Yes, we're just checking that the function runs ok, and the grids returns the expected values (no NaNs, etc). It helps when we do major refactoring, to ensure nothing goes wrong.

[Esteban82]

It sounds to me that you could use the static server instead.
https://static.generic-mapping-tools.org/

This is its readme file:

# This directory is meant to be static, i.e., we only place unchanged data
# sets that our test scripts need to use so that we do not get massive test
# failures when the topography is updated a bit.

Right now it doesn't have any gravimetric data, but I think we could add it.