Parameterizes test_se_message_echo over an editable list of TEST_ROOMs.

This lets us easily modify the list of rooms that this test is run against, each with independent pass/fail results. Ideally, this could be used to test that everything is always working against all three hosts, but there isn't a suitable test account for that purpose.

On the main branch, this should only include Charcoal HQ for now, but I'll also be adding the SO sandbox on my test branch.