autotest: run BatteryFailsafe at default speedup

[peterbarker]

Summary

Remove pointless speed limiting in battery failsafe testing

Classification & Testing (check all that apply and add your own)

• Checked by a human programmer
• Non-functional change
• No-binary change
• Infrastructure change (e.g. unit tests, helper scripts)
• Automated test(s) verify changes (e.g. unit test, autotest)
• Tested manually, description below (e.g. SITL)
• Tested on hardware
• Logs attached
• Logs available on request

This has been soak-tested in my own fork.

● BatteryFailsafe savings, measured on real CI runners (from the 120-sample soak, not local):

  ┌───────────────────────┬──────────────────────────────────┬─────────────────────────────────────────┬──────────────────────────┐
  │                       │      Before (SIM_SPEEDUP=4)      │         After (default speedup)         │          Saved           │
  ├───────────────────────┼──────────────────────────────────┼─────────────────────────────────────────┼──────────────────────────┤
  │ Unloaded runner       │ 322 s (10-run production median) │ 89.5 s (median of 15, spread 88.7–92.8) │ ~232 s ≈ 3.9 min         │
  ├───────────────────────┼──────────────────────────────────┼─────────────────────────────────────────┼──────────────────────────┤
  │ Under 2 CPU stressers │ —                                │ 105.1 s                                 │ still ~217 s vs baseline │
  └───────────────────────┴──────────────────────────────────┴─────────────────────────────────────────┴──────────────────────────┘

Description

BatteryFailsafe pins SIM_SPEEDUP to 4 for its entire duration, making it the fourth most expensive test in all of CI at ~320 wall-seconds per run. The override has been carried through several refactors with no recorded rationale; the original 2013 test did not set it.

Battery failsafe timing is entirely simulation-time: the voltage comes from SIM_BATT_VOLTAGE and BATT_LOW_TIMER counts simulation seconds, so there is no wall-clock coupling that would require a reduced speedup (unlike the GCS failsafe tests, where the test framework's wall-paced heartbeats justify one). Remove the override so the test runs at the suite default.

[khancyr]

if that has been tested, go for it, the analysis is correct