test: fix test-http-pipeline-flood

The number of connections achieved by the test can vary by platform
and by machine. Lowering the acceptance threshold so that the
test passes on Windows.
dns: verify argument is valid function in resolve

Don't use argument as callback if it's not a valid callback function.
Throw a valid exception instead explaining the issue.

Adds to #7070 ("DNS — Throw meaningful error(s)").
dns: validate arguments in resolver

Mitigate C++-land assertion error, add test accordingly.

Fix #7070