Fix scheduler/twilight bug, add test for dusk startup

This is in response to to discussion in this thread https://indilib.org/forum/general/10463-scheduler-not-stopping-for-twilight-constraint.html?start=12#76347

Previously, in some geographies (e.g. Australia) the scheduler didn't wait for twilight to start up, even though the twilight restriction was checked.

I added a scheduler test (testTwilightStartup in test_ekos_scheduler_ops.cpp) for San Francisco and Melbourne showing that (previously) the scheduler worked for SF but did not honor the restriction in Melbourne.

@edejouhanet found the issue in schedulerjob.cpp / calculateDawnDusk(). I incorporated his fix into this MR.

Now the test passes for both cities.