fix: Use "ir.testrun.org" as default iroh relay

[Septias]

close #5591

[Septias]

The JSON-RPC python tests are failing because we can't set [cfg(test)] there. Maybe we can compile json-rpc-server with RUSTFLAGS='--cfg test=1', or we can just add an internal config option to use iroh relays. The third option is to check for some env var like IROH_RELAY=b1.testrung.org to choose the iroh relay. I don't really have an opinion on what is best @link2xt

[Hocuri]

What's the status of this PR? The code looks good to me, but I didn't really follow iroh developments.

I rebased in order to test whether the tests pass.

[Septias]

There is still discussion in the related issue

[link2xt]

ir.testrun.org simply does not exist and currently used chatmail server points to iroh.testrun.org:4443 from metadata so fallback is not tested by CI.