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

WORKSPACE

@@ -4,27 +4,44 @@ load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 
 http_archive(
     name = "io_bazel_rules_go",
+    sha256 = "e88471aea3a3a4f19ec1310a55ba94772d087e9ce46e41ae38ecebe17935de7b",
     urls = [
         "https://storage.googleapis.com/bazel-mirror/github.com/bazelbuild/rules_go/releases/download/v0.20.3/rules_go-v0.20.3.tar.gz",
         "https://github.com/bazelbuild/rules_go/releases/download/v0.20.3/rules_go-v0.20.3.tar.gz",
     ],
-    sha256 = "e88471aea3a3a4f19ec1310a55ba94772d087e9ce46e41ae38ecebe17935de7b",
 )
 
-load("@io_bazel_rules_go//go:deps.bzl", "go_rules_dependencies", "go_register_toolchains")
+load("@io_bazel_rules_go//go:deps.bzl", "go_register_toolchains", "go_rules_dependencies")
 
 go_rules_dependencies()
 
 go_register_toolchains()
 
+##### Protobuf
+_PROTOBUF_VERSION = "3.6.1"
+
+http_archive(
+    name = "com_google_protobuf",
+    patch_args = ["-p1"],
+    patches = [
+        "//bazel:protobuf_deps_3_6_1.patch",
+    ],
+    strip_prefix = "protobuf-" + _PROTOBUF_VERSION,
+    urls = [
+        "https://github.com/protocolbuffers/protobuf/archive/v" + _PROTOBUF_VERSION + ".tar.gz",
+    ],
+)
 
 ##### GRPC
 _GRPC_VERSION = "1.15.1"
 
 http_archive(
     name = "com_github_grpc_grpc",
-    patches = ["//bazel:grpc_deps_1_15_1.patch"],
     patch_args = ["-p1"],
+    patches = [
+        "//bazel:grpc_deps_1_15_1.patch",
+        "//bazel:grpc_deps_1_15_1_02.patch",
+    ],
     strip_prefix = "grpc-" + _GRPC_VERSION,
     urls = ["https://github.com/grpc/grpc/archive/v" + _GRPC_VERSION + ".tar.gz"],
 )