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/protobuf_deps_3_6_1.patch

@@ -0,0 +1,24 @@
+diff --git a/protobuf.bzl b/protobuf.bzl
+index 78f19c621..3b6d183b9 100644
+--- a/protobuf.bzl
++++ b/protobuf.bzl
+@@ -130,7 +130,7 @@ proto_gen = rule(
+         "protoc": attr.label(
+             cfg = "host",
+             executable = True,
+-            single_file = True,
++            allow_single_file = True,
+             mandatory = True,
+         ),
+         "plugin": attr.label(
+@@ -266,8 +266,8 @@ def internal_gen_well_known_protos_java(srcs):
+   Args:
+     srcs: the well known protos
+   """
+-  root = Label("%s//protobuf_java" % (REPOSITORY_NAME)).workspace_root
+-  pkg = PACKAGE_NAME + "/" if PACKAGE_NAME else ""
++  root = Label("%s//protobuf_java" % (native.repository_name())).workspace_root
++  pkg = native.package_name() + "/" if native.package_name() else ""
+   if root == "":
+     include = " -I%ssrc " % pkg
+   else: