[application_agent test] Updated use of protos

New Protobuf version marks messages as final. Modifying test to
accomodate this.

Bug: 1144985
Change-Id: If211677e537321b557b53579f3645604484962a4
Reviewed-on: https://chromium-review.googlesource.com/c/openscreen/+/2522291
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Commit-Queue: mark a. foltz <mfoltz@chromium.org>
diff --git a/cast/receiver/application_agent_unittest.cc b/cast/receiver/application_agent_unittest.cc
index 18191e7..c89a451 100644
--- a/cast/receiver/application_agent_unittest.cc
+++ b/cast/receiver/application_agent_unittest.cc
@@ -67,14 +67,13 @@
   StaticCredentialsProvider creds_;
 };
 
-class TestAuthChallengeMessage : public CastMessage {
- public:
-  TestAuthChallengeMessage() {
-    const auto result = ParseFromString(
-        ReadEntireFileToString(GetTestDataSubdir() + "/auth_challenge.pb"));
-    OSP_CHECK(result);
-  }
-};
+CastMessage TestAuthChallengeMessage() {
+  CastMessage message;
+  const auto result = message.ParseFromString(
+      ReadEntireFileToString(GetTestDataSubdir() + "/auth_challenge.pb"));
+  OSP_CHECK(result);
+  return message;
+}
 
 class FakeApplication : public ApplicationAgent::Application,
                         public MessagePort::Client {