fake-vm: Fix tests with deadline signal reaching wrong test threads
Since internal tests in fake-vm create a thread to mock an actor, the deadline signal may reach one of these actor threads instead of the application thread. More than failing to interrupt the application thread, in some cases this causes the test to deadlock, which is both inefficient testing and bad for automated testing and CI.
To fix this, instead of creating a thread to mock an actor, just fork a new process using the nix crate API.