Add flag for overriding API environment in E2E tests

[dlon]

Fix DES-1051.

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This change is 

[linear]

DES-1051 Control environment for `test-manager` with flags

[MarkusPettersson98]

Reviewed 4 of 6 files at r1, 1 of 2 files at r2, 1 of 1 files at r3, 1 of 1 files at r4, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved

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test/test-manager/src/main.rs line 77 at r4 (raw file):

        display: bool,

        /// API and conncheck environment to use, such as "mullvad.net" or "stagemole.eu".

⛏️ It is not that necessary to list mullvad.net and stagemole.eu in the comment, since it will be printed as part of the help text when invoking test-runner run-tests --help

$ cargo run --bin test-manager -- run-tests --help
Spawn a runner instance and run tests
..
Options:
      ..
      --mullvad-host <MULLVAD_HOST>
          API and conncheck environment to use, such as "mullvad.net" or "stagemole.eu". The domain name will be prefixed with "api." and "ipv4.am.i."

          [possible values: mullvad.net, stagemole.eu, devmole.eu]
      ..

Code quote:

 /// API and conncheck environment to use, such as "mullvad.net" or "stagemole.eu".

[MarkusPettersson98]

Reviewed 2 of 2 files at r5, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved