The issue:

Not all storage engines support the replSetResizeOplog command (see SERVER-38356). In an attempt to maintain the same starting state for each iteration of the test (by maintaining the same oplog size), the replSetResizeOplog command is used but fails on the ephemeralForTest storage engine, which does not support the command.

Proposed fixes:

A solution is to remove the line that tries to maintain uniformity at the start of each test. Since this is a part of the auth suite, it is unlikely that the particular size of the oplog even matters.

Another solution is to introduce the isWiredTigerOrInMemory() check - only when it is true, then issue the replSetResizeOplog command. Otherwise, just drop and create a new oplog. I think this is the best solution.