BUG: ArbitraryContrast is not tested

[larsoner]

In working on streamlining some stuff in #765 I noticed that ArbitraryContrast does not seem to be used in any tests:

$ git grep "contrasts = "
...
mne_bids_pipeline/steps/sensor/_99_group_average.py:        decoding_contrasts = []
mne_bids_pipeline/tests/configs/config_ERP_CORE.py:    contrasts = [("unrelated", "related")]
mne_bids_pipeline/tests/configs/config_ERP_CORE.py:    contrasts = [("response/incorrect", "response/correct")]
mne_bids_pipeline/tests/configs/config_ERP_CORE.py:    contrasts = [("response/right", "response/left")]  # contralateral vs ipsi
mne_bids_pipeline/tests/configs/config_ERP_CORE.py:    contrasts = [("stimulus/deviant", "stimulus/standard")]
mne_bids_pipeline/tests/configs/config_ERP_CORE.py:    contrasts = [("stimulus/right", "stimulus/left")]  # Contralteral vs ipsi
mne_bids_pipeline/tests/configs/config_ERP_CORE.py:    contrasts = [("stimulus/face/normal", "stimulus/car/normal")]
mne_bids_pipeline/tests/configs/config_ERP_CORE.py:    contrasts = [("stimulus/target", "stimulus/non-target")]
mne_bids_pipeline/tests/configs/config_ds000117.py:contrasts = [
mne_bids_pipeline/tests/configs/config_ds000246.py:contrasts = [("deviant", "standard")]
mne_bids_pipeline/tests/configs/config_ds000248_base.py:contrasts = [("Visual", "Auditory"), ("Auditory/Right", "Auditory/Left")]
mne_bids_pipeline/tests/configs/config_ds000248_no_mri.py:contrasts = [("Auditory/Right", "Auditory/Left")]
mne_bids_pipeline/tests/configs/config_ds001810.py:contrasts = [("61450", "61511")]
mne_bids_pipeline/tests/configs/config_ds001971.py:contrasts = [("AdvanceTempo", "DelayTempo")]
mne_bids_pipeline/tests/configs/config_ds003392.py:contrasts = [("incoherent", "coherent")]
mne_bids_pipeline/tests/configs/config_eeg_matchingpennies.py:contrasts = [("raised-left", "raised-right")]

You can't see ds000117 here but it's a list of tuple of str. None of these use the TypedDict that is ArbitraryContrast, so it's likely either broken or going to break.

@hoechenberger or @agramfort do you want to open a quick PR to change one of the tests/configs/* to use an ArbitraryContrast in some meaningful way? If not I can add a potentially bad/useless example somewhere...