[v9_18] doth system test - decrease HTTP listener quota size

This commit reduces the size of HTTP listener quota from 300 (default) to 100 so that it would make hitting any global limits in case of running multiple tests in parallel in multiple containers unlikely.

This way the need in opening many file descriptors of different kinds (e.g. client side connections and pipes) gets significantly reduced while the required code paths are still verified.

Backport of !6898 (merged)

Closes #3596 (closed)