Add test that makes sure connecting works without the API

[MarkusPettersson98]

This PR adds a test to verify that the app can connect to a VPN server and get working internet even if the API is unavailable. As long as the user has managed to log in to the app, the rest should just work.

---

This change is 

[linear]

DES-586 Write test that makes sure connecting works without the API

[dlon]

Reviewed 6 of 6 files at r1, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @MarkusPettersson98)

---

test/test-manager/src/tests/helpers.rs line 25 at r1 (raw file):

use test_rpc::{package::Package, AmIMullvad, ServiceClient};

/// TODO: Document

TODO

---

test/test-manager/src/tests/helpers.rs line 342 at r1 (raw file):

/// # Note
/// This is just a thin wrapper around [`ServiceClient::set_daemon_environment`] which also
/// invalides the old [`MullvadProxyClient`].

Nit: invalidates

[dlon]

Reviewed 4 of 4 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved