Introduce upgrade command unit tests

This change introduce a basic unit test for the `linkerd upgrade`
command. Given a mock k8s client with linkerd-config and
linkerd-identity-issuer objects, it validates the rendered yaml output
against an expected file.

To enable this testing, most of the logic in the top-level upgrade
command has been moved down into a `validateAndBuild` method.

TODO:
- test individual functions around mutating options, flags, configs, and
  values
- enable reading the install information from a manifest rather than k8s

Part of #2637

Signed-off-by: Andrew Seigner <[email protected]>