For some reason I can't explain, since 2b0870084b
we got test failures on OS X, as the clipboard had the old value before waiting
for the change, the new (correct) value after waiting for it, but never
actually emitted 'changed'.
We could just re-check the contents after the timeout, but that'd mean we wait
1s for every test where this weird thing happens.
Instead, we poll the clipboard for every 100ms as long as the timeout (1s)
hasn't passed, and return as soon as it has the correct contents.
eebed7a5a7