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

bazel/grpc_deps_1_15_1_02.patch

@@ -0,0 +1,48 @@
+diff --git a/bazel/cc_grpc_library.bzl b/bazel/cc_grpc_library.bzl
+index 3288565714..6bfcd653f5 100644
+--- a/bazel/cc_grpc_library.bzl
++++ b/bazel/cc_grpc_library.bzl
+@@ -1,6 +1,6 @@
+ """Generates and compiles C++ grpc stubs from proto_library rules."""
+ 
+-load("//:bazel/generate_cc.bzl", "generate_cc")
++load("//bazel:generate_cc.bzl", "generate_cc")
+ 
+ def cc_grpc_library(name, srcs, deps, proto_only, well_known_protos, generate_mocks = False, use_external = False, **kwargs):
+   """Generates C++ grpc classes from a .proto file.
+diff --git a/bazel/generate_cc.bzl b/bazel/generate_cc.bzl
+index ae747aa42c..2f14071f92 100644
+--- a/bazel/generate_cc.bzl
++++ b/bazel/generate_cc.bzl
+@@ -83,7 +83,7 @@ _generate_cc = rule(
+     attrs = {
+         "srcs": attr.label_list(
+             mandatory = True,
+-            non_empty = True,
++            allow_empty = False,
+             providers = ["proto"],
+         ),
+         "plugin": attr.label(
+diff --git a/bazel/grpc_build_system.bzl b/bazel/grpc_build_system.bzl
+index 73147bf3ac..e8c392aa93 100644
+--- a/bazel/grpc_build_system.bzl
++++ b/bazel/grpc_build_system.bzl
+@@ -24,6 +24,9 @@
+ #
+ 
+ # The set of pollers to test against if a test exercises polling
++
++load("//bazel:cc_grpc_library.bzl", "cc_grpc_library")
++
+ POLLERS = ['epollex', 'epollsig', 'epoll1', 'poll', 'poll-cv']
+ 
+ def if_not_windows(a):
+@@ -95,8 +98,6 @@ def grpc_proto_plugin(name, srcs = [], deps = []):
+     deps = deps,
+   )
+ 
+-load("//:bazel/cc_grpc_library.bzl", "cc_grpc_library")
+-
+ def grpc_proto_library(name, srcs = [], deps = [], well_known_protos = False,
+                        has_services = True, use_external = False, generate_mocks = False):
+   cc_grpc_library(