autopkgtest 5.28 now available across several series

[Heather Ellsworth]

Hi all,

A few months ago, there was some discussion[0][1] about needing the 
autopkgtest package to provide the -a architecture option, in all 
supported releases. While this had been available on the infrastructure, 
it was not available in the various series' pockets.

I worked alongside the QA team to provide this patch in the lunar 
autopkgtest, and backport that to other supported releases with an SRU 
Exception in each case.

Now we have the same version of autopkgtest in jammy, kinetic, lunar, 
and mantic (not focal yet). This new autopkgtest contains a couple of 
key new features:

## Cross-architecture Testing

autopkgtest did not have the -a ARCH capability needed to permit 
cross-architecture testing. This is notably useful for i386 local 
testing and was noted to be missing since the focal release.

Now you can create an amd64 VM where autopkgtest can run on an i386 package.

There was an existing patchset for this functionality from ~3 years ago 
that needed tweaking to provide the -a option.

## Pocket Pinning

Ubuntu has switched to NotAutomatic: yes for the proposed pocket.
This means that by default packages from -proposed won't be installed 
unless explicitly requested.

This feature is meant to make it easier for end users to keep -proposed
enabled (for SRU verification or individual package installation)
without manually adding a pin to avoid pulling in all of -proposed.

However when running autopkgtests with --apt-pocket=proposed we do want
to pull in all the pocket, so let's always give it Pin-Priority: 500.
This is similar to what Launchpad does with the buildds.

Note: This change does not really treat -proposed in a special way.
Any pocket will get Pin-Priority: 500. It makes sense to keep the tool
behavior consistent, and we're not aware of cases where this could be
cause issues.

Cheers,
Heather

[0] https://lists.ubuntu.com/archives/ubuntu-devel/2022-December/042397.html
[1] https://lists.ubuntu.com/archives/ubuntu-devel/2023-January/042403.html