autopkgtest: test binNMU's

During transitions, until now we have let binNMU's migrate without testing. For very long, the argument was that we don't want to throw away smooth-updates behavior by blocking transitions. But we don't need to block the library from migrating if it's smooth-updateable, what we want to block is the rebuilds if they cause regressions.

It turns out that adding that support is not very complicated as by adding an apply_srcarch_policy_impl method to the AutopkgtestPolicy already gets is called at exactly the right moment. All we need to do is add the right checks that are different than for source uploads.

Closes: #944458

First two commits are the same as in !122 (merged)