test: backoff client connection rates

We were being very aggressive in our connection creations, resulting
in the pipeline flood detection to drop us. Relax how fast we're
creating these connections so the gc can run all its tests.
src: node.cc use isolate->ThrowException

Environment doesn't have ThrowException, we meant isolate here.

Introduced in commit
75adde07f9a2de7f38a67bec72bd377d450bdb52.