[Nowax] Reset FNodes between unit tests

[WhoSoup]

The sim is set up to use engine.fnodes to store the simulated nodes but this is never reset between tests, meaning new tests just keep appending to the end of fnodes. This leads to errors like databases being closed (since the nodes are shut down).

Similarly, the "quit" channel that StartSim uses was never reopened after being closed from a previous test.

Both of these changes are experimental with possible unintentional consequences.