Use sub-processes in integration testing

Spawn master, slave, and clusterctl in separate processes in the
clusterio integration tests and leave the test master and slave process
available for the whole test duration to allow plugins to use them as
well.  This removes the need for reimplementing the startup logic in the
test code and makes the existing startup code covered by tests.

test/missing.js

@@ -4,4 +4,3 @@
 require('lib/manager/modManager');
 require('lib/npmPostinstall.js');
 require('lib/typedef');
-require('../clusterctl.js');