Add TriggerTester methods to check that timers are set

TriggerTester exposes a fireIfShouldFire(BoundedWIndow) method. This is used to prompt a call to the trigger with the current state of the trigger tester (Input Watermarks, elements present, etc), and see if the trigger should fire. This is sufficient to test shouldFire of the TriggerStateMachine under test, but does not test that the onElement method sets an appropriate timer to cause such an interrogation to occur.

Imported from Jira BEAM-694. Original Jira may contain additional context. Reported by: jkff.