Fix error when trying to execute multi-model ReplayMachine

[Teekivi]

Executing a ReplayMachine created from a multi-model machine ran into the following error as soon as the execution left the first model (output from executing replayMultiModelMachine without the fix):

[main] ERROR o.g.core.machine.SimpleMachine.walk - No path generator is defined

org.graphwalker.core.machine.MachineException: org.graphwalker.core.machine.MachineException: No path generator is defined

	at org.graphwalker.core.machine.SimpleMachine.walk(SimpleMachine.java:147)
	at org.graphwalker.core.machine.SimpleMachine.getNextStep(SimpleMachine.java:102)
	at org.graphwalker.core.machine.ReplayMachineTest.compareMachineWithReplayMachine(ReplayMachineTest.java:59)
	at org.graphwalker.core.machine.ReplayMachineTest.replayMultiModelMachine(ReplayMachineTest.java:53)
        ...
Caused by: org.graphwalker.core.machine.MachineException: No path generator is defined
	at org.graphwalker.core.machine.SimpleMachine.hasNextStep(SimpleMachine.java:270)
	at org.graphwalker.core.machine.SimpleMachine.switchContext(SimpleMachine.java:179)
	at org.graphwalker.core.machine.SimpleMachine.chooseSharedContext(SimpleMachine.java:201)
	at org.graphwalker.core.machine.SimpleMachine.takeNextStep(SimpleMachine.java:190)
	at org.graphwalker.core.machine.SimpleMachine.walk(SimpleMachine.java:140)
	... 28 more

This commit contains a test demonstrating the issue and my fix for it, each in its own commit.

[KristianKarl]

Very nice PR! The separate commits for the test and fix is much appreciated.
Looks good to me