tests: run check-code with warnings and maintain a whitelist
check-code has fine warning checks, but they are a bit noisy and nobody used
them.
Now the warnings will be run in the test suite, where a list of accepted
warnings will be maintained.
Those who introduce or touch a line with a warning will now have to update the
whitelist ... or fix the warning.
workingfilectx.date = (1000, 0)
ASCII : Gr?ezi!
Latin-1 : Grüezi!
UTF-8 : Grüezi!