test: Add capability to disable RPC timeout in functional tests.

Summary:

Modifies the existing --factor flag to --timeout-factor to better express intent.
Adds rules to disable timeout if --timeout-factor is set to 0.
Modfies --timeout-factor help doc to inform users about this feature.

docs: Add notes on how to diasble rpc timeout in functional tests while attatching gdb.

This is a backport of Core PR18986

Test Plan: test/functional/test_runner.py --timeout-factor=0

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9125