The test should make sure the set is healthy, with a primary (and all other nodes are up) before adding roles/users.

    var rstest = new ReplSetTest({name: name, nodes: 1, nodeOptions: {}});

    rstest.startSet();
    rstest.initiate();

    m0 = rstest.nodes[0];
...
    rstest.add();
    rstest.reInitiate();

    rstest.getPrimary().getDB("db1").createRole({...

After the reInitiate call m0 might no longer be primary, or be stepping down. There should be a write(w:2)/await or something to ensure that the set has a primary is healthy (all members).