The new test Docker image has a newer gnupg which does not behave the
same way, handling invalid passwords. This type of error is simulated
using an ICP error in the pinentry protocol.
This makes it possible to run e.g. "yadm -Y foo introspect repo
2>/dev/null" and get an empty output instead of getting the error
message about foo not being fully qualified.
The new test system is written with py.test. These tests are more
comprehensive, run faster by an order of magnitude, and are far more
maintainable. The tests themselves conform to PEP8.