test: add more test coverage (#80)

* build: Update mmcore to 12.0.0

MMEventCallback callbacks become always asynchronous.

Tests for callbacks now use threading.Event to wait (with a timeout) for
the async call.

(Assisted by Claude Code; any errors are mine.)

* change pattern

* more tests

* comments

* better

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Co-authored-by: Mark A. Tsuchida <matsuchida@wisc.edu>

Author: tlambert03