Make the test network exponential backoff multiplier configurable

In #20597 (moved), we hard-coded a multiplier of 2 (exponent 3) in test networks, and a multiplier of 3 (exponent 4) by default. These should probably be configurable using a torrc option.

This task should be performed after we modify the exponential backoff distributions in #20604 (moved) and #20605 (moved).

Follow-up to #20499 (moved), #20597 (moved), and #20534 (moved).