Update retry to improve machine placement on read and introduce internal/juju unit tests with mocking

[hmlanigan]

Description

Resolves TODO from #359 where minimal application import test was failing by improving retry on ReadApplication to wait for machines to exists when required. Does not apply to CAAS models, not subordinate applications.

Adds unit tests for internal/juju utilizing the stretchr test package. Currently there is coverage for ReadApplicationWithRetryOnNotFound. More can be added as we go. A new make target has been added to run them.

This involved refactoring parts of internal/juju to allow for mocks to be used by introducing new local interfaces and wrappers on the api clients to talk to juju.

Stretchr has some notable differences with the way tests are written in github.com/juju/juju. Notably: suite.Required() is equivalent to Assert() and suite.Assert() is equivalent to Check().

Type of change

• Change in tests (one or several tests have been changed)
• Bug fix (non-breaking change which fixes an issue)

QA steps

terraform {
  required_providers {
    juju = {
      version = ">= 0.9.0"
      source  = "juju/juju"
    }
  }
}

provider "juju" {
}

resource "juju_model" "testmodel" {
  name = "testme"
}

resource "juju_application" "application_three" {
  model = juju_model.testmodel.name
  units = 2
  charm {
    name = "juju-qa-test"
  }
}

resource "juju_application" "application_one" {
  model = juju_model.testmodel.name
  units = 0
  charm {
    name = "ntp"
  }
}

resource "juju_integration" "juju-info" {
  model = juju_model.testmodel.name
  application {
    name = juju_application.application_three.name
    endpoint = "juju-info"
  }
  application {
    name = juju_application.application_one.name
    endpoint = "juju-info"
  }
}

Additional notes

JUJU-5427

[Aflynn50]

Good amount of testing!

[Aflynn50]

This error message could be a bit confusing to a user, does "output" mean something to them?

[cderici]

This is an amazing start for the unit tests! Code looks good, QA went well 👍

[hmlanigan]

Fixed a test and squashed in the lint change with the major commit - required a force.