test: remove magic numbers in test-gc-http-client-onerror

Remove magic numbers (500, 10, 100) from the test. Instead, detect when
GC has started and stop sending requests at that point.

On my laptop, this results in 16 or 20 requests per run instead of 500.

Fixes: https://github.com/nodejs/node/issues/23089

PR-URL: https://github.com/nodejs/node/pull/24943


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>