chore(ci): use nextests groups to isolate some tests

[divagant-martian]

Description

After a lot of debugging we realized the integrations tests (gc tests, for
example) run with swarm discovery on since they are not technically using test
iroh, so they bypass the cfg that prevents it from being turned on. This causes
the test_local_swarm_discovery test to be flooded with nodes from other
tests, making it flaky. Isolating the test should make it no longer flaky
anymore

Breaking Changes

n/a

Notes & open questions

We should still create a test that creates a bunch of nodes and makes sure they
all can find each other within reasonable time. There is still reason to
believe swarm discovery is slow, particularly in linux, but we should address
it in a test under our countrol, not via the non-deterministic nature of
concurrent tests in ci.

Change checklist

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

[Arqu]

Agree on the tests to be added.

This looks to work as intended. Ran cargo nextest show-config test-groups to confirm and got what was expected:

 Finished `test` profile [unoptimized + debuginfo] target(s) in 1m 35s
group: run-in-isolation (max threads = 1)
  * override for default profile with filter 'test(::run_in_isolation::)':
      iroh-net:
          discovery::local_swarm_discovery::tests::run_in_isolation::test_local_swarm_discovery
          ```

[flub]

oh, this cool. maybe there are other things we could use this for. A bit of a shame the only kind of marker you have is the name, but better than nothing.