Add test for spm build. (#26178)

Package.swift

@@ -22,6 +22,8 @@ let package = Package(
   dependencies: [
     .package(
       name: "abseil",
+      //TODO(yulinliang): Firebase abseil is too old, update the dependency
+      //when abseil repo supports SPM.
       url: "https://github.com/firebase/abseil-cpp-SwiftPM.git",
       .revision("05d8107f2971a37e6c77245b7c4c6b0a7e97bc99")
     ),
@@ -140,6 +142,13 @@ let package = Package(
         .unsafeFlags(["-Wno-module-import-in-extern-c"]),
       ]
     ),
+    .testTarget(
+      name: "build-test",
+      dependencies: [
+        "gRPC-cpp",
+      ],
+      path: "test/spm_build"
+    ),
   ],
   cLanguageStandard: .gnu11,
   cxxLanguageStandard: .cxx11

test/spm_build/test.cc

@@ -0,0 +1,29 @@
+/*
+ *
+ * Copyright 2021 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+#include "grpcpp/channel.h"
+#include "grpcpp/client_context.h"
+#include "grpcpp/completion_queue.h"
+#include "grpcpp/create_channel.h"
+#include "grpcpp/generic/generic_stub.h"
+#include "grpcpp/grpcpp.h"
+#include "grpcpp/impl/codegen/grpc_library.h"
+#include "grpcpp/support/byte_buffer.h"
+#include "grpcpp/support/status.h"
+#include "grpcpp/support/status_code_enum.h"
+#include "grpcpp/support/string_ref.h"