Make test_onroad stressful

Now that test_onroad is reliable, it's time to add some things to really stress the system:
* lots of disk load: openpilot shouldn't rely on a responsive disk. catches things like reading params in a real-time loop
* change the system time: catch things like https://github.com/tinygrad/tinygrad/commit/cda34ccadf07181422dfb9ff38470d1454cfff1e

Start by locating the reliable test_onroad entry point and reviewing how it currently exercises the system. Define stress scenarios for heavy disk load and system-time changes, then verify that test_onroad remains reliable and catches failures such as reading parameters in a real-time loop or depending on wall-clock time.

Written by the indexing model from the issue text.