After this commit the use of interval in tests is combined with
take(n).onBackpressureBuffer(n) to ensure emissions don't fail if the
fixed rate is exceeded (e.g. on slow CI server).
Tests that verify N number of items followed by verifyOnComplete()
should set the number of emissions to N.
Tests that verify N number of items followed by thenCancel() should
set the number of buffered to an arbitrary number greater than N.
1653a32a04