tests: Speed up threaded toggle notify test unless -m slow is passed

On a fast laptop, this test currently takes about 7s to run, which is a significant portion of the overall test suite time.

On a slower CI machine, especially running the test under valgrind, the test can time out.

There’s no need to always run so many iterations: we run the tests under CI so often that it’s likely a failure will eventually be hit (if there is a bug) even with fewer iterations. We also now run the tests once a week with -m slow, so the original iteration count will also still be used then.

Signed-off-by: Philip Withnall pwithnall@gnome.org