autotest: run external AHRS tests at reduced speedup

The external AHRS drivers parse their serial stream on a real-time
thread.  At ArduPlane's default autotest speedup (100x) that thread
cannot keep the simulated GPS within its health window on a loaded CI
runner: a given wall-clock scheduling stall maps to 100x as much
sim-time, pushing the GPS frame delta past the is_healthy() thresholds.
This produced intermittent "GPS 1: not healthy" arm failures in
InertialLabsEAHRS (and is a latent risk for the other EAHRS tests that
share fly_external_AHRS).

Lower the speedup to 20x for the duration of fly_external_AHRS so the
parse thread keeps the GPS healthy.  Use context_set_speedup() inside a
context_push()/context_pop() pair so the SIM_SPEEDUP parameter and the
self.speedup timeout scaler stay in sync and are restored together,
even if the test raises.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: peterbarker

Tools/autotest/arduplane.py

@@ -3106,6 +3106,13 @@ def fly_generic_mission(self, filename, mission_timeout=60.0, strict=True,
     def fly_external_AHRS(self, sim, eahrs_type,
                           dist_to_final_wp_threshold_m: float | None = None):
         """Fly with external AHRS"""
+        # The external AHRS drivers parse their serial stream on a
+        # real-time thread.  At the default plane speedup (100x) that
+        # thread cannot keep the simulated GPS within its health window
+        # on a loaded CI runner, producing intermittent "GPS 1: not
+        # healthy" arm failures.  Run these tests at a reduced speedup so
+        # the thread keeps up.
+        self.context_set_speedup(20)
         self.customise_SITL_commandline(["--serial4=sim:%s" % sim])
 
         self.set_parameters({