test(iroh-relay, netcheck): move tests to use local relays

[divagant-martian]

Description

Removes netcheck's dependency on staging relays, using locally served ones instead. This is necessary to move netcheck out without needing to find an appropriate place for the staging values to prevent circular dependecies

Breaking Changes

• RelayMap now can be created with an iterator of Arcs directly.

Notes & open questions

• a test was removed since it doesn't test properties of netcheck, it just connects to the staging relay
• iroh-relay/src/server/testing.rs was added by mistake in the split pr, so the deletions are not really picked by rust and all new code should be considered.. well, new.

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• Tests if relevant.
• All breaking changes documented.