fix: test race condition

[Integralist]

We're seeing data race issues in CI:
https://github.com/domainr/epp2/actions/runs/8330541688/job/22795438262?pr=22

There are a few different asynchronous layers within these epp2 tests...

• echoServer is run in a goroutine and reads from t.
• echoServer also runs a separate goroutine that writes to t.
• The caller of echoServer also runs multiple goroutines to write to t.

The changes in this PR should resolve the data race issue.

Instead of passing around the *testing.T I'm using channels to handle the coordination/synchronization of error messages.

[cee-dub]

The only possibly-contentious item is the error comparison that is known not to be compatible with TinyGo.

IMO, we should develop this as if TinyGo will gain that feature by the time Domainr and TinyGo need to ship these things together.

[Integralist]

All changes have been made. I'm stepping away from the keyboard for 30mins and I'll merge when I'm back.