There are many tests of committed reads that all have to perform a similar check to determine if the test can be run against the current storage engine (e.g. jstests/replsets/read_committed.js and jstests/replsets/read_committed_no_shapshots.js). Not only is the code duplicated, but it's also pretty hard to read. We should move it into a library such as jstests/replsets/rslib.js.