Windows CI tests regularly failing due to issues with cached remote files for sph2grd

[maxrjones]

We seem to be semi-regularly getting failures on windows-latest - Python 3.7 / NumPy 1.18 with :
..\tests\test_sph2grd.py::test_sph2grd_outgrid FAILED [ 87%]
..\tests\test_sph2grd.py::test_sph2grd_no_outgrid FAILED [ 87%]
due to issues with the remote file.

Originally posted by @meghanrjones in #1242 (comment)

Full error message at https://github.com/GenericMappingTools/pygmt/runs/4099608763?check_suite_focus=true#step:11:578:

____________________________ test_sph2grd_outgrid _____________________________

    def test_sph2grd_outgrid():
        """
        Test sph2grd with a set outgrid.
        """
        with GMTTempFile(suffix=".nc") as tmpfile:
>           result = sph2grd(
                data="@EGM96_to_36.txt", outgrid=tmpfile.name, spacing=1, region="g"
            )

C:\Miniconda3\envs\pygmt\lib\site-packages\pygmt\tests\test_sph2grd.py:16: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
C:\Miniconda3\envs\pygmt\lib\site-packages\pygmt\helpers\decorators.py:586: in new_module
    return module_func(*args, **kwargs)
C:\Miniconda3\envs\pygmt\lib\site-packages\pygmt\helpers\decorators.py:726: in new_module
    return module_func(*args, **kwargs)
C:\Miniconda3\envs\pygmt\lib\site-packages\pygmt\src\sph2grd.py:75: in sph2grd
    lib.call_module("sph2grd", arg_str)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <pygmt.clib.session.Session object at 0x000002E91CE65370>
Error:  [ERROR]: Cannot find file C:/Users/runneradmin/.gmt/cache/EGM96_to_36.txt
Error:  [ERROR]: File C:/Users/runneradmin/.gmt/cache/EGM96_to_36.txt not found
[Session pygmt-session (1719)]: Error returned from GMT API: GMT_FILE_NOT_FOUND (16)
___________________________ test_sph2grd_no_outgrid ___________________________

    def test_sph2grd_no_outgrid():
        """
        Test sph2grd with no set outgrid.
        """
>       temp_grid = sph2grd(data="@EGM96_to_36.txt", spacing=1, region="g")

C:\Miniconda3\envs\pygmt\lib\site-packages\pygmt\tests\test_sph2grd.py:27: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
C:\Miniconda3\envs\pygmt\lib\site-packages\pygmt\helpers\decorators.py:586: in new_module
    return module_func(*args, **kwargs)
C:\Miniconda3\envs\pygmt\lib\site-packages\pygmt\helpers\decorators.py:726: in new_module
    return module_func(*args, **kwargs)
C:\Miniconda3\envs\pygmt\lib\site-packages\pygmt\src\sph2grd.py:75: in sph2grd
    lib.call_module("sph2grd", arg_str)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <pygmt.clib.session.Session object at 0x000002E91CEC3BB0>
module = 'sph2grd'
args = '@EGM96_to_36.txt -GC:\\Users\\RUNNER~1\\AppData\\Local\\Temp\\pygmt-1jo39m30.nc -I1 -Rg'

    def call_module(self, module, args):
        """
        Call a GMT module with the given arguments.
    
        Makes a call to ``GMT_Call_Module`` from the C API using mode
        ``GMT_MODULE_CMD`` (arguments passed as a single string).
    
        Most interactions with the C API are done through this function.
    
        Parameters
        ----------
        module : str
            Module name (``'coast'``, ``'basemap'``, etc).
        args : str
            String with the command line arguments that will be passed to the
            module (for example, ``'-R0/5/0/10 -JM'``).
    
        Raises
        ------
        GMTCLibError
            If the returned status code of the function is non-zero.
        """
        c_call_module = self.get_libgmt_func(
            "GMT_Call_Module",
            argtypes=[ctp.c_void_p, ctp.c_char_p, ctp.c_int, ctp.c_void_p],
            restype=ctp.c_int,
        )
    
        mode = self["GMT_MODULE_CMD"]
        status = c_call_module(
            self.session_pointer, module.encode(), mode, args.encode()
        )
        if status != 0:
>           raise GMTCLibError(
                f"Module '{module}' failed with status code {status}:\n{self._error_message}"
            )
E           pygmt.exceptions.GMTCLibError: Module 'sph2grd' failed with status code 16:
Error:      sph2grd [ERROR]: Libcurl Error: Timeout was reached
Error:      sph2grd [ERROR]: File C:/Users/runneradmin/.gmt/cache/EGM96_to_36.txt was not found
Error:      sph2grd [ERROR]: Cannot find file C:/Users/runneradmin/.gmt/cache/EGM96_to_36.txt
Error:      sph2grd [ERROR]: File C:/Users/runneradmin/.gmt/cache/EGM96_to_36.txt not found

C:\Miniconda3\envs\pygmt\lib\site-packages\pygmt\clib\session.py:500: GMTCLibError

[weiji14]

I found the problem, there's a missing @ sign and comma here:

pygmt/pygmt/helpers/testing.py

Lines 170 to 172 in 4b80f3c

	"S90W180.earth_age_05m_g.nc" # Specific grid for 05m test
	# Other cache files
	"@EGM96_to_36.txt",

so two files weren't being downloaded:

gmtwhich [ERROR]: Cannot find file S90W180.earth_age_05m_g.nc@EGM96_to_36.txt
gmtwhich [ERROR]: File S90W180.earth_age_05m_g.nc@EGM96_to_36.txt not found!

This might explain why your earth_age_05m tests were failing #1471 (comment). I'll just tell @willschlitzer to add the comma at #1615 since he needs to re-run the cache anyway.