InputTestDriver sleeps exactly until input loop emits all the test data

In order to avoid sleeping long, specify expected emits and then
InputTestDriver continuously sleeps very short so that input loop will emit all the test data.
Can specify only the length of expected emits and assert emits after InputTestDriver#run
Change variable name

t0         ->
started_at
Add a comment about expected_emits_length
Merge pull request #397 from itiut/avoid-long-sleep

InputTestDriver sleeps exactly until input loop emits all the test data