binderRpcTest: threadpool race leak

SessionWithIncomingThreadpoolDoesntLeak is flaking at 0.02%.
This is another small fix to avoid noise in the tests. It's
not great, but we know it works because OnewayCallExhaustion
uses this solution and does not flake.

Bug: 285206261
Test: binderRpcTest --gtest_filter="*SessionWithIncomingThreadpoolDoesntLeak*"
Change-Id: I8a1678e9633db71cb4afda2f0f5c108ca784f803