Document DUMMY_INTERRUPT on Testing Using DummyBackends page

[leventerevesz]

As the Testing applications using the DummyBackends documentation page suggests, the DummyBackends are quite handy for testing application code.

If the application uses push-type inputs, it's necessary to trigger interrupts in the DummyBackend. This page does not mention how to trigger an interrupt.

Please include a paragraph on how to use the DUMMY_INTERRUPT virtual registers.

[killenb]

MSK internal ticket: https://redmine.msktools.desy.de/issues/12888