autotest: budget FRSkyPassThroughStatustext in simulated time

The test waits for a statustext to reach it over the FRSky passthrough
link, and allowed "7 * self.speedup" simulated seconds for it.  That
scales the wrong way.  Measured, with an unlimited budget, the simulated
time needed to receive the wanted text is

    speedup    1     2     5    10    20   100
    needs    58.4  53.9  48.7  39.4  39.9   9.9 s
    allowed   7    14    35    70   140   700  s

The requirement falls as the speedup rises, because what has to happen
first is that the queue ahead of our text drains, and at a high speedup
far less simulated time passes while that happens in wall clock.  The
budget rose instead, so it handed out 71 times what was needed at the
default speedup - no assertion at all - and less than was needed at
anything below about 7.  It failed every time at --speedup=5, three
passes out of three in an overnight sweep, and had only 1.8x margin at
--speedup=10.

Allow a fixed 150 simulated seconds: 2.6x the slowest measured, and
still a real bound at the default speedup.  Passes at speedups 1, 5, 10
and 100.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Tools/autotest/vehicle_test_suite.py

@@ -15130,7 +15130,15 @@ switch value'''
 
         received_frsky_texts = []
         last_len_received_statustexts = 0
-        timeout = 7 * self.speedup # it can take a *long* time to get these messages down!
+        # the queue has to drain before the text we are looking for
+        # reaches us, and how long that takes is best measured in
+        # simulated time: 58s at speedup 1, 49s at 5, 39s at 10 and 20,
+        # 10s at 100 - it falls as the speedup rises.  Scaling the budget
+        # by the speedup therefore had it backwards, handing out 700s
+        # where 10 was needed and 35s where 49 was, so this failed every
+        # time at --speedup=5.  Allow a fixed 150s, comfortably above the
+        # slowest measured and still an assertion at the default speedup.
+        timeout = 150
         while True:
             self.drain_mav()
             now = self.get_sim_time_cached()