Adding e2e testing relating to config

[EduardGomezEscandell]

The launcher modifies some config files in WSL. These tests verify this happens.

Feature commits:

• 96dc60f tests that systemd is enabled and that the fixes the launcher applies to to sysusers service work.
• 8acfd93 tests that the upgrade policy is set according to the distro name
• 5f1d754 tests that we used the right rootfs link. This test will have to be updated every time there is new release. I didn't want to automate it because the point of this test is to test this very automation.

Cleanup commits:

• 7cf259e. This commit is best viewed without whitespace diffs. I enclosed each set of assertions in their own scope to avoid bleeding variables from one test to the next one. This commit makes big enough changes that merging the latter cleanup commits with the previous ones is too much of a hassle.

Later commits are bugfixes and smaller changes.

[CarlosNihelton]

I think we have room for improvement here.

[CarlosNihelton]

LGTM!

Great job with the function split-up! It looks gorgeous!