Does this task subsume T7419?

More or less. Broadly, the plan here is:

• Harbormaster "owns" lint and unit test results in the future.
• Arcanist, Harbormaster, and third-party test integrations upload results to Harbormaster, either as part of a build or in some as-yet-unspecified way that's more like "here are some results for this thing, they aren't really part of a build per se". Not sure if that'll be a special case or a real build with a special flag or what.
• Differential pulls results in from Harbormaster and renders them.

This task doesn't really cover that responsibility change, but it will happen when Harbormaster moves forward.

After D14190, you can do this with a vanilla "Command" build step that runs something like this:

$ arc unit --everything --conduit-uri <api-token> --target ${target.phid}

This is relatively not-that-terrible, although I think it's probably still worthwhile to have a custom step for this which is a thin wrapper around the command step.

The need to have a bare API token here is a little sketchy. Putting it in Passphrase and exposing it as a build variable would be one step better. Doing Conduit over SSH might be even cleaner (T550).