SwitchOnFirst stress test: investigate flaky test

[simonbasle]

   [FAILED] reactor.core.publisher.FluxSwitchOnFirstStressTest.InboundSubscribeAndOutboundCancelAndInboundCompleteStressTest
    (JVM args: [-XX:+UnlockDiagnosticVMOptions, -XX:+WhiteBoxAPI, -XX:-RestrictContended, -Dfile.encoding=UTF-8, -Duser.country, -Duser.language=en, -Duser.variant, -XX:-TieredCompilation, -XX:+StressLCM, -XX:+StressGCM])
             Observed state   Occurrences   Expectation  Interpretation                                              
  0, 0, 1, 1, 1, 1, 0, 0, 1        10,230     FORBIDDEN  No default case provided, assume FORBIDDEN                  
  0, 0, 1, 1, 1, 1, 2, 0, 1       408,131    ACCEPTABLE  Cancellation happened as the earliest event. firstValue i...
  0, 0, 1, 2, 2, 0, 1, 1, 0        60,230    ACCEPTABLE  Inbound got second request, delivered onNext('value') and...
  0, 0, 1, 2, 2, 1, 2, 1, 0        65,350    ACCEPTABLE  Inbound got second request, delivered onNext('value') but...

This is occurring from time to time on main branch, and disappears on subsequent runs.

What's concerning is the number of FORBIDDEN state occurrences (10K), which might indicate an actual bug in the operator 🤔