Wait for mirrored operations in mirror_reads.js

There are no barriers between the tests in jstests/noPassthrough/mirror_reads.js, which allows some stats to leak between tests.

In particular, we may trigger a series of reads in a test (from here) and wait for the number of mirrored requests to exceed a minimum threshold (in here). Once the minimum is reached, we check the number of mirrored requests against a maximum and continue with the next test. However, some of the mirrored requests may still be in progress, causing the next test to fail.

To address this, we must ensure all mirrored requests are resolved before returning from each test, ensuring no mirroring operation spans over multiple tests.

Acceptance Criteria: 

The particular test has proper isolation between test cases.