Please allow queueing tests automatically after builds successfully finished

[Balint Reczey]

In the current workflow the Core Dev has to upload the package to the Bileto PPA, regenerate the diff, then wait until the package is built and only then press the button to queue the tests.

The build step may take several hours and push the tests to the next workday instead of having them run during the night automatically. This is suboptimal due to the extra delay and also due to using the test resources when they are more crowded during the work day.

Tests should not be triggered automatically by default however, because there may be extra tests needed to make sure that the package is worth triggering all the autopkgtests. I proposed adding an extra option to "Lander Signoff", something like "Approved when built" to queue the tests when all the builds are successful.