test: mock internet access in test_upgrade

[aciba90]

Proposed Commit Message

test: mock internet access in test_upgrade

Fixes: #5211

Additional Context

Test Steps

Checklist

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[holmanb]

Thanks for the fix @aciba90!

Unfortunately this might crop again since I don't think we test anything like this. Perhaps an lxd integration test which takes down the network and then runs the unittests would help us prevent this from happening again?

[TheRealFalcon]

@holmanb , good idea. I opened #5220 for this.