Add a test verifying number of writes per RPC is reasonable

Mostly this is code copied from bm_fullstack and rephrased as a test.
I'm resisting the urge to unify it however, as I expect this code will evolve differently over time.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 81f3b2d..1c4d355 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -9479,6 +9479,36 @@
 endif (gRPC_BUILD_TESTS)
 if (gRPC_BUILD_TESTS)
 
+add_executable(writes_per_rpc_test
+  test/cpp/performance/writes_per_rpc_test.cc
+  third_party/googletest/src/gtest-all.cc
+)
+
+target_include_directories(writes_per_rpc_test
+  PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}
+  PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/include
+  PRIVATE ${BORINGSSL_ROOT_DIR}/include
+  PRIVATE ${PROTOBUF_ROOT_DIR}/src
+  PRIVATE ${ZLIB_ROOT_DIR}
+  PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/third_party/zlib
+  PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/third_party/gflags/include
+  PRIVATE third_party/googletest/include
+  PRIVATE third_party/googletest
+)
+
+target_link_libraries(writes_per_rpc_test
+  grpc++_test_util
+  grpc_test_util
+  grpc++
+  grpc
+  gpr_test_util
+  gpr
+  ${_gRPC_GFLAGS_LIBRARIES}
+)
+
+endif (gRPC_BUILD_TESTS)
+if (gRPC_BUILD_TESTS)
+
 add_executable(boringssl_aes_test
   third_party/googletest/src/gtest-all.cc
 )