Improve notify center tests.

[maleadt]

• local notify centers are synchronous, so we shouldn't run the loop. this speeds up testing
• the Darwin notify center is asynchronous, so we should run the loop. do so longer to make the test more robust

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (44a5aae) 69.68% compared to head (1c93d16) 69.68%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #28 +/- ## ======================================= Coverage 69.68% 69.68% ======================================= Files 9 9 Lines 630 630 ======================================= Hits 439 439 Misses 191 191 ```

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