[Binder Transport] Flush ExecCtx in e2e test (#32971)

WireWriter implementation schedules actions to be run by `ExecCtx`. We
should flush pending actions before destructing
`end2end_testing::g_transaction_processor`, which need to be alive to
handle the scheduled actions. Otherwise,
we get heap-use-after-free error because the testing fixture
(`end2end_testing::g_transaction_processor`) is destructed before all
the scheduled actions are run.

This lowers end2end binder transport test failure rate from 0.23% to
0.15%, according to internal tool that runs the test for 15000 times
under various configuration.

test/core/transport/binder/end2end/end2end_binder_transport_test.cc

@@ -50,6 +50,7 @@ class End2EndBinderTransportTest
   ~End2EndBinderTransportTest() override {
     server_->Shutdown();
     service_.reset();
+    exec_ctx.Flush();
     delete end2end_testing::g_transaction_processor;
   }