[CrOS Tether] Create proto_test_util.cc/h and move CreateTestDeviceStatus() there.

chromeos/components/tether/BUILD.gn

Index: chromeos/components/tether/BUILD.gn
diff --git a/chromeos/components/tether/BUILD.gn b/chromeos/components/tether/BUILD.gn
index 3a5c33b242745f7048443571ba9c66c409a37eeb..ec274f333feea67b750449b0055978ef3386ad92 100644
--- a/chromeos/components/tether/BUILD.gn
+++ b/chromeos/components/tether/BUILD.gn
@@ -116,6 +116,8 @@ static_library("test_support") {
     "mock_local_device_data_provider.h",
     "mock_tether_host_response_recorder.cc",
     "mock_tether_host_response_recorder.h",
+    "proto_test_util.cc",
+    "proto_test_util.h",
   ]
 
   public_deps = [
@@ -125,6 +127,7 @@ static_library("test_support") {
   deps = [
     "//base",
     "//chromeos",
+    "//chromeos/components/tether/proto",
     "//components/cryptauth",
     "//testing/gmock",
     "//testing/gtest",