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/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++",
+    ],
+)