Clarify blacklist code, drop blacklisted measures on ingress, cc #238
Add thorough blacklisting tests, cc #238.
Use slightly more uniform / less hacky helper functions, cc #238.
Small import and style cleanup.

Stop trying to avoid "module/function under test" from being imported on
the module scope. We have so many import dependencies that this isn't
actually working.
Move test station definition outside the loop.
Correctly set creation date of measures in tests.

Without this all measures where created `now`. Which meant the location
update code could get confused, as its ordering by creation date. With
the exact same creation date (one second resolution), it would sometimes
get the older measure and use it twice, instead of using the latest measure.