I came up with this work in progress!

# Starting "epmd"
epmd_path = System.find_executable("epmd")
port = Port.open({:spawn_executable, epmd_path}, [ ])
os_pid = Keyword.get(Port.info(port), :os_pid)

# Configuring a "shutdown hook" to stop epmd after everything is done.
# (I'm not particularly proud of this implementation, honestly)
System.at_exit(fn _ -> System.shell("kill -TERM #{os_pid}") end)

# This is a bit of an exageration just to compute a name
{:ok, now} = DateTime.now("Etc/UTC")
now = DateTime.to_unix(now)
cipher = '#{now}'
  |> Enum.map(fn c -> c + 49 end)
  |> List.to_string()

# Turn this node into a distributed node
{:ok, _} = Node.start(String.to_atom("#{cipher}@testhost"))

# Finally, start the tests
ExUnit.start()

For some reason, I was unable to make correct use of Port.close/1, so I used the kill command to stop epmd. Obviously, this implementation will fail in windows.