BUG: Wrong output for adjacency=False

[larsoner]

Regarding the API, there is already a test test_output_equiv to which I added the threshold parameter to be tested. However, I also noticed the False value for adjacency was not tested and indeed there are problems here:

• adjacency=False runs only for 1D inputs (which I think is not expected)
• for 1D input, the output is always "indices" whatever is out_type

I guess it is rarely used... Should this be corrected in the same or another PR?

Originally posted by @nfourcau in https://github.com/mne-tools/mne-python/pull/12609#discussion_r1603338916