Add Pin-Priority to pockets added via --apt-pocket

Ubuntu is switching to NotAutomatic: yes for the proposed pocket: the change is already operative on >= Lunar and under consideration for older stable releases. This means that by default packages from -proposed won't be installed unless explicitly requested.

The change 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.