tests: adapt GTest smoke-tests to MockDDS annotated payload format (seq:payload)

Lts/DDS/DdsUnitTestsPlaceholder/ddsGenericDataTests_gtest.cpp

@@ -11,5 +11,11 @@ TEST(GenericSmokeGTest, MockDDSPublishReceive) {
   MockDDS::clear_all();
   MockDDS::publish("topic1", "payload42");
   auto r = MockDDS::take_last("topic1");
-  EXPECT_EQ(r, "payload42");
+  // format is "<seq>:<payload>"
+  auto pos = r.find(':');
+  ASSERT_NE(pos, std::string::npos);
+  auto seq = std::stoul(r.substr(0, pos));
+  auto payload = r.substr(pos + 1);
+  EXPECT_EQ(payload, "payload42");
+  EXPECT_EQ(seq, 1UL);
 }