clean up "test-privileged", "test-unprivileged" users

[davepacheco]

This change cleans up a few things related to the built-in users:

• The test users ("test-privileged" and "test-unprivileged") are now Silo Users rather than built-in users. (This enables the rest of the cleanup below.)
• We no longer create a Silo User for any built-in users. (We used to create a Silo User for each built-in user).
• Built-in users no longer have a Silo associated with them. (Recall that built-in users are identities used by Nexus to limit its own privileges. They're not principals in the usual sense and they're not supposed to be associated with any Silo in particular.)
• It's no longer possible to authenticate to the external API as a built-in user. IAM: support assigning roles on the Fleet #1051 recently extended "spoof" to treat users as Silo Users by default, but allow specifying a built-in user. Now you can't specify a built-in user.

There are a few more things that would be nice to do but we can't do quite yet:

• In principle, it'd be nice if we didn't create the test users (test-privileged and test-unprivileged) in Nexus, but rather in the test suite. However, test-privileged is the only identity that's ever really capable of doing anything -- including creating other users. So we need it to be created by Nexus during startup -- the test suite has no creds it could use to do this itself. The eventual answer here is the bootstrapping of the initial Silo and IdP integration discussed in RFD 234. (The test users are also used by the console spoof login, but that's not really a blocker for this. It's really just the lack of a real bootstrapping process.)
• The default Silo is used for a lot less now. It'd be nice to remove it, but we can't for the same reason -- we need a real bootstrapping process to get things started.

[plotnick]

This seems like an unambiguous improvement. Built-in users being in a Silo was confusing, and the test users not being in a Silo was also confusing. Patches that reduce confusion are the best kind of patches!

[davepacheco]

Thanks for the reviews!