test: fix sequential/test-async-wrap-getasyncid

Previously, this test would contain a DNS query that timed out after 60 seconds, thus occupying one of the parallel test slots for that period.

Fix that by creating a new channel for that request, and cancelling it immediately.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

test