fix: add test util to wait for healthy piri

[alanshaw]

Aims to fix:

=== RUN   TestFXSpaceContentRetrieve/space/content/retrieve
    ucan_fx_test.go:106: 
            Error Trace:    /Users/runner/work/piri/piri/pkg/service/retrieval/ucan_fx_test.go:106
            Error:          Received unexpected error:
                            sending message: doing HTTP request: Get "http://localhost:49232/piece/bafkreibmtmy3p773cf7uj24hfg2dhniov45xv4p54cs4oe3nxhm7p3ov5e": dial tcp [::1]:49232: connect: connection refused

Possibly because the server is not ready to accept connections yet.

[frrist]

I think there are a couple more tests that would benefit from the addition of this, would be nice to add this call there, but non-blocking.