Originally Posted by GinFizz

Finland? How many curveballs are you going to throw us ...?

But okay, let's work with that. So it seems that for Finland the specific rules allow for either:
#1 two tests on the same day at different testing facilities, or
#2 two tests separated by 24 hours using the same testing facility,

where both tests must be within 48 hours of departure and the second test must be within 24 hours of departure.

So on paper at least a 15 hour transit might work - but the timing is extremely tight given that you will need to use option #1 (two tests same day at different places), and the Finnair flight to China leaves at 13:25. That means that you need to upload the information to the health-code app at the latest I would say 10:00 am to be safe, so you will need the result from both tests by that time.

The open question here is how strictly "same day" should be interpreted. To be safe I would interpret this as literally the same calendar day (not "within 24 hours"). If you have a 15 hour transit time that means you will be arriving late at night the previous day.

So in that case you will need to pass through immigration (enter Finland) then go to one location to do test #1, then go to do test #2 at a different test location, doing both tests before midnight that day. Or wait until the next day (after midnight) then take the two tests.

In the first case you will need to find two places that stay open very late. For the second case you will need places that are either open 24 hours, or open very early and can provide results in few hours (e.g. one test at 06:00 and another at 07:00 with a 3 hour turn around for the results).

This just seem unnecessarily risky, if even possible. Even if there is one test place at the airport, you still need a second test centre to use option #1. And in either case you will need to enter Finland, and so there is no disadvantage that I can see to spending a longer time "in transit" to allow time to get the tests sorted.