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

Lts/DDS/DdsUnitTestsPlaceholder/ddsGenericDataTests_gtest.cpp

@@ -1,4 +1,5 @@
 #include <gtest/gtest.h>
+#include "mocks/MockDDS.h"
 
 TEST(GenericSmokeGTest, BasicStringOps) {
   std::string s = "MiXeD";
@@ -6,8 +7,9 @@ TEST(GenericSmokeGTest, BasicStringOps) {
   EXPECT_EQ(s, "mixed");
 }
 
-TEST(GenericSmokeGTest, VariantLike) {
-  // simulate basic core logic check
-  int v = 42;
-  EXPECT_EQ(v + 1, 43);
+TEST(GenericSmokeGTest, MockDDSPublishReceive) {
+  MockDDS::clear_all();
+  MockDDS::publish("topic1", "payload42");
+  auto r = MockDDS::take_last("topic1");
+  EXPECT_EQ(r, "payload42");
 }