tests: fix APT news tests for bionic

[lucasmoura]

Why is this needed?

On Bionic, the distro-info-data package version is bigger than what we have on ESM. Therefore, if we upgrade when attached, we will get the ESM version, due to the pin file. After we detach and run apt upgrade, distro-info-data will then be upgraded. Since our tests required that we have no packages to upgrade when checking the apt news messages, we are now adding an additional apt upgrade step after we detach just to get rid of this issue.

Test Steps

Check that the APT News bionic tests are now passing

Checklist

• [ ] I have updated or added any unit tests accordingly
• [x] I have updated or added any integration tests accordingly
• [ ] Changes here need to be documented, and this was done in:

Does this PR require extra reviews?

• [ ] Yes
• [x] No

[github-actions[bot]]

Jira: This PR is not related to a Jira item. (The PR title does not include a SC-#### reference)

GitHub Issues: No GitHub issues are fixed by this PR. (No commits have Fixes: #### references)

Launchpad Bugs: No Launchpad bugs are fixed by this PR. (No commits have LP: #### references)

Documentation: The changes in this PR do not require documentation changes.

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