[JUJU-1200] Wait for CRM to be active before migrating

[jack-w-shaw]

Model migration tests using cross-model relations were often failing because the migration would be started before the CRM was active. Resolve this by waiting until the CRM is active in effected tests

Also fix some shfmt lint failures

Checklist

• Code style: imports ordered, good names, simple structure, etc
• [ ] Comments saying why design decisions were made
• [ ] Go unit tests, with comments saying what you're testing
• Integration tests, with comments saying what you're testing
• [ ] doc.go added or updated in changed packages

QA steps

./main.sh -v -s 'test_model_config,test_model_multi,test_model_metrics' model

[wallyworld]

We need to fix the root cause. Papering over the cracks by adding a wait to the test doesn't help users who try and run a migration "too soon". The root cause is that export model fails if the connection is not yet ready. So there's 2 things to fix in juju: 1. juju dump-model (which calls export model) needs to handle this case gracefully; 2. migration needs to fail with a user friendly error or else handle the case gracefully if we thing that it is feasible that the migrated model will still pick up where things were left off

[wallyworld]

This PR #14414 fixes the underlying issue highlighted by the test failure - a 5 year old bug. We still need the wait, but the error being surfaced is now correctly determined as part of the migration prechecks and not via an unexpected internal error.

[jack-w-shaw]

/merge