While reading read_write_concern_defaults_application.js, I happened to notice that it creates roles "foo" and "bar", grants role "bar" to role "foo", and then tests revokeRolesFromRole by revoking the "foo" role from itself!

In other words, the "foo" role is revoked from the "foo" role. The test was probably intended to revoke "bar" role from the "foo" role. The test still fulfills its purpose but it doesn't test revokeRolesFromRole as it's used in the wild.