Allow RunScenarios to spawn in-process workers

This allows us to get back to single binary tests where appropriate, which will help in-depth profiling efforts.
I've built this atop my smoke_test changes as they inspired me to get this done.
diff --git a/test/cpp/qps/driver.h b/test/cpp/qps/driver.h
index 7a81b70..bfa0e68 100644
--- a/test/cpp/qps/driver.h
+++ b/test/cpp/qps/driver.h
@@ -56,7 +56,8 @@
                            const grpc::testing::ServerConfig& server_config,
                            size_t num_servers,
                            int warmup_seconds,
-                           int benchmark_seconds);
+                           int benchmark_seconds,
+                           int spawn_local_worker_count);
 
 }  // namespace testing
 }  // namespace grpc