Fix races between background snapshot saving and check for upgrades

Originally created by @intrigeri on #6592 (Redmine)

When running the automated test suite against 0.22.1~rc1, I’ve noticed that some scenarios sometimes fail, apparently due to the background snapshot having been made at a wrong time, resulting in the Tails Upgrader “Cannot check for upgrades” dialog that our test suite does not expect.

The likely fix would be to wait for the check for upgrades to have completed, before saving the background snapshot.

Feature Branch: bugfix/6592-fix-races-with-check-for-upgrades

Related issues

• Blocked by #6593 (closed)