bin/test-run should support testing a proxy image

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## Feature Request

### What problem are you trying to solve?

When making large changes to the proxy, it's often desirable to use the integration tests in this repository to validate that the changes in the proxy behave correctly in conjunction with the control plane. Currently, this takes some effort, as it's necessary to rebuild the docker images for the proxy, the proxy-init container, the control plane, and the CLI binaries, in order to use `bin/test-run` to run the integration tests.

### How should the problem be solved?

Now that the `linkerd upgrade` command exists, it's possible to deploy Linkerd from a release tag and then set the tag for the proxy image to use. As an alternative to taking a path to a `linkerd` binary, it would be nice if the `bin/test-run` script took a tag for a proxy image. It could then deploy the control plane from the latest edge release, and run `linkerd upgrade -v $PROXY_TAG` to roll out the specified proxy tag for testing, and run the tests once all proxies have been upgraded. This would make integration testing for the proxy much easier.

Start with bin/test-run and trace how it currently selects binaries and deploys the proxy, control plane, and proxy-init images. Review the linkerd upgrade command usage described in the issue, then verify the script can accept a proxy image tag, deploy the release control plane, upgrade proxies to that tag, and run integration tests after the upgrade completes.

Written by the indexing model from the issue text.