fix(tests): bind to unique ports in ibc tests

[conorsch]

Describe your changes

Follow-up to #4747. Noticed the new tests were flaking across multiple runs. Turns out they were fighting for port binds, as described already in #4517. Let's just use a unique port pair for the MockRelayers and call it a day for now.

Issue ticket number and link

Refs #4517.

Checklist before requesting a review

• If this code contains consensus-breaking changes, I have added the "consensus-breaking" label. Otherwise, I declare my belief that there are not consensus-breaking changes, for the following reason:

  tests only

[conorsch]

Here's an example of a failing test run: https://github.com/penumbra-zone/penumbra/actions/runs/10515320975/job/29135123028 with error text:

--- STDERR:              penumbra-app::ibc_handshake ibc_handshake ---
thread 'ibc_handshake' panicked at crates/core/app/tests/common/ibc_tests/node.rs:120:47:
grpc server returned an error: Os { code: 98, kind: AddrInUse, message: "Address already in use" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Error: could not connect to grpc server

Caused by:
    0: transport error
    1: error trying to connect: tcp connect error: Connection refused (os error 111)
    2: tcp connect error: Connection refused (os error 111)
    3: Connection refused (os error 111)