tests: add MockDDS and use it in GTest smoke-tests to simulate publish/subscribe flows

Lts/DDS/DdsUnitTestsPlaceholder/ddsFunctionTopicTests_gtest.cpp

@@ -1,4 +1,5 @@
 #include <gtest/gtest.h>
+#include "mocks/MockDDS.h"
 #include <map>
 
 TEST(FunctionTopicSmokeGTest, Containers) {
@@ -7,3 +8,10 @@ TEST(FunctionTopicSmokeGTest, Containers) {
   m["two"] = 2;
   EXPECT_EQ(m["two"], 2);
 }
+
+TEST(FunctionTopicSmokeGTest, MockDDSTopicAsSignal) {
+  MockDDS::clear_all();
+  MockDDS::publish("func/topic", "evt:1");
+  auto val = MockDDS::take_last("func/topic");
+  EXPECT_EQ(val, "evt:1");
+}