Make the test network exponential backoff multiplier configurable
In legacy/trac#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 legacy/trac#20604 (moved) and legacy/trac#20605 (moved).
Follow-up to legacy/trac#20499 (moved), legacy/trac#20597 (moved), and legacy/trac#20534 (moved).