Improve test disk usage and I/O in tests

[larsoner]

I/O companion to #14265, a few low-hanging fruit for speeding up tests further:

• change some _TempDir to tmp_path
• copy fewer files (e.g. for subjects_dir_tmp, only copy fsaverage when needed)
• add some raw/event crops when the test can still test reasonably with less data
• cache reads of small data and reuse them with .copy in fixtures

Changes drafted by Claude Fable 5 and reviewed/iterated on by me