Event getters: Test differences between filter and action context

[Luashine]

Related: #136

Water 4/25/24:
Is there any other functional difference between TriggerRegisterUnitEvent and TriggerRegisterFilterUnitEvent other than latter allows to specify an additional filter?

Water 5/1/24
that's curious, too, when you register
TriggerRegisterFilterUnitEvent(trig, passenger, EVENT_UNIT_LOADED, filter)
the GetFilterUnit in the filter and GetTriggerUnit in the fired trigger will both be the passenger

but when you do
TriggerRegisterPlayerUnitEvent(trig, whichPlayer, EVENT_PLAYER_UNIT_LOADED, filter)
the GetFilterUnit in the filter will be the transporter and GetTriggerUnit in the fired trigger will be the passenger

This is probably an issue for much later, when there's automated testing to compare between:

1. TriggerRegisterPlayerUnitEvent
2. TriggerRegisterUnitEvent
3. TriggerRegisterFilterUnitEvent

[shuen4]

TriggerRegisterUnitEvent mostly = TriggerRegisterFilterUnitEvent with null filter

second one is just blizzard fault (or bug)
they didnt overload GetFilterUnit method (default return unit2, which is transporter)