Learn English – is it correct to say “When it started to rain, I was in the open air.”

They are all good.

My choice would be On arrival

They all give the sense of the first thing that we were asked to do after we'd arrived was complete the placement test.

So I think it's whichever floats your boat.

It depends on how exactly the state of the brightness of the lights needs to match the pre-dimmed state, and what level the brighter setting is. I.e. if you just want the lights set brighter than they currently are, or to be set to the highest brightness setting, you can be less specific in your instruction.

My choice for a less specific instruction would be (as mentioned in several comments):

turn up the lights / turn the lights up - if the lights were not previously set lower/if the lights being previously set lower is irrelevant
turn the lights back up - if the lights were previously dimmed

The second of these gives the implication of returning to a previous state, but doesn't explicitly say that it must be to exactly the previous state.
Turn is especially suitable if it is a rotating dimmer switch

If there is a specific setting for the lights to be returned to (e.g. going from 1/4 brightness to 1/2 brightness, or changing to half of the lights at full brightness, half of the lights at 1/2 brightness) then you need to be more specific about it. Words like reset, restore, return or redo may be appropriate.
Note, however, that these terms do not specify that the lights are to be brightened.

Also note that just saying:

do the lights

may well be understood in the context. It can also be used to mean dim the lights; for example before a slideshow you could say "do the lights" to mean dim the lights and afterwards also say "do the lights" to mean make the lights bright again.