MRG, MAINT: Speed up tests

[larsoner]

Looking at some of the slowest ones on the Azure test that keeps failing (avoiding 3D viz tests for now) on master I get these timings:

75.15s total   mne/preprocessing/tests/
57.95s total   mne/beamformer/tests/
28.77s total   mne/io/fiff/
21.70s total   mne/io/fieldtrip/
22.52s total   mne/io/egi/
14.10s total   mne/io/eeglab/

On this PR, which mostly uses crop and reduction of redundant test cases to speed things up, I get:

55.78s total   mne/preprocessing/tests/
47.89s total   mne/beamformer/tests/
24.87s total   mne/io/fiff/
13.01s total   mne/io/fieldtrip/
 7.62s total   mne/io/egi/
 9.59s total   mne/io/eeglab/

So at least some incremental progress locally, and hopefully differences will be even better on CIs.

[agramfort]

thx @larsoner