mirrored_reads.js doesn't wait for all mirrored reads to resolve

When we wait for read mirroring to complete here, we never check that our "sent" value is equal to our "resolved" value. This means that occasionally, we have achieved the desired level of mirroring but there are still operations being mirrored to secondaries. This becomes a problem when we run 0% mirroring tests on slow variants, because even a single late operation breaks the statistical bounds in the test.