Grab passphrase on demand during tests

To support testing various different snap versions in the test suite, we should bring back support to grab the trust establishment passphrase from stdout. This is what [we did originally](https://github.com/canonical/microcloud/pull/383/changes#diff-439a2a1d53a67caf300c4abd74eb1a3561bcfe682eef6450d94ef4bcee815285R180).

By doing this, we don't require a specific binary which is built with testing mode but instead can run the binary inside the snap and simply grab its passphrase for the joiners.
This also requires `TEST_CONSOLE=(0|1)` environment variable to be present again in the regular snap binary to be able to switch to non-interactive input mode.

Also this requires backport to `v2` so we can start testing `2/candidate` in the weekly pipeline.