`StartNode` runs a naked goroutine, so it's impossible to test against
it in a way that will reliably produce contained test failures when
assertions are hit on the `(*node).run` goroutine.
This commit introduces a harness that we can use in tests to wrap
this goroutine and allow it to defer errors to `*testing.T`.
Note that tests of `Node` still need to be architected carefully
since it's easy to produce a deadlock in them should things not
go exactly as planned.
Signed-off-by: Tobias Grieger <tobias.b.grieger@gmail.com>
