`TestMembersLocal` -> "member presence" test relies on parallel side-effects

[ShadowJonathan]

I found this while investigating why the runtime.Dendrite skipif was present;

The TestMembersLocal test, specifically, the "Existing members see new members' presence" test, relies on this simultaneously-executed room join to pass.

When ran in isolation, this test fails, as that line is not executed to make this test pass, it times out on syncing.