Fix ABI L1b/L2 time dimension causing issues with newer xarray

[djhoese]

Pointed out originally by @TAlonglong on slack. @yufeizhu600 also helped debug this one.

It seems in newer versions of xarray that the scalar variables in a NetCDF variable like t in the ABI L1b files is no longer treated as a dimension (self.nc.dims) and is only considered a coordinate variable. The ABI base reader specifically checked for it existing in the dims before renaming it to time to be more compatible with the rest of Satpy. This PR fixes this by checking the .dims and the .coords for a t variable. Continuing to check in .dims may be redundant, but I think it is useful in the case there is a declared dimension in a NetCDF file but no corresponding coordinate variable defined in the file.

To test this I had to update the ABI L1b/L2 and GLM L2 tests to stop using a "FakeDataset" object when a normal xarray Dataset object will do just fine.

• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy

[coveralls]

Coverage increased (+0.02%) to 87.296% when pulling e9c1a53 on djhoese:bugfix-abi-t-dimension into 2f9501e on pytroll:master.

[codecov]

Codecov Report

Merging #1036 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1036      +/-   ##
==========================================
+ Coverage   87.29%   87.29%   +<.01%     
==========================================
  Files         188      188              
  Lines       28742    28715      -27     
==========================================
- Hits        25089    25067      -22     
+ Misses       3653     3648       -5

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_glm_l2.py	95.16% <100%> (+0.28%)	⬆️
satpy/readers/abi_base.py	91.6% <100%> (+3.05%)	⬆️
satpy/tests/reader_tests/test_abi_l2_nc.py	93.93% <100%> (+0.09%)	⬆️
satpy/tests/reader_tests/test_abi_l1b.py	96.87% <100%> (-0.35%)	⬇️

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[mraspaud]

LGTM