Allow to run cpp tests under grpc/tests/ using bazel. (#6040)

This is the first step to upgrade grpc dependency to the latest version.

- Patch protobuf 3.6.1. and grpc 1.15.1 to fix build errors when using the latest Bazel version (3.4.1).
- Add grpc/tests:grpc_test. One can kick off tests in Bazel by calling `bazel test grpc/tests/...`.
- Add missing build targets in tests/BUILD in order to support grpc/tests/BUILD

grpc/README.md

@@ -22,6 +22,12 @@ the GRPC libraries for this to compile. This test will build using the
 5. `cmake -DFLATBUFFERS_BUILD_GRPCTEST=ON -DGRPC_INSTALL_PATH=${GRPC_INSTALL_PATH} -DPROTOBUF_DOWNLOAD_PATH=${PROTOBUF_DOWNLOAD_PATH} ..`
 6. `make`
 
+For Bazel users:
+
+```shell
+$bazel test src/compiler/...
+```
+
 ## Running FlatBuffer gRPC tests
 
 ### Linux
@@ -29,3 +35,9 @@ the GRPC libraries for this to compile. This test will build using the
 1. `ln -s ${GRPC_INSTALL_PATH}/lib/libgrpc++_unsecure.so.6 ${GRPC_INSTALL_PATH}/lib/libgrpc++_unsecure.so.1`
 2. `export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:${GRPC_INSTALL_PATH}/lib`
 3. `make test ARGS=-V` 
+
+For Bazel users:
+
+```shell
+$bazel test tests/...
+```

grpc/src/compiler/BUILD

@@ -104,4 +104,4 @@ cc_library(
     deps = [
         "//:flatbuffers",
     ],
-)
+)

grpc/tests/BUILD

@@ -0,0 +1,17 @@
+cc_test(
+    name = "grpc_test",
+    srcs = [
+        "grpctest.cpp",
+        "message_builder_test.cpp",
+    ],
+    copts = ["-Itests"],
+    # This is required.
+    linkstatic = 1,
+    deps = [
+        "//tests:monster_test_cc_fbs",
+        "//tests:monster_test_grpc",
+        "//tests:test_assert",
+        "//tests:test_builder",
+        "@com_github_grpc_grpc//:grpc++",
+    ],
+)