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

Lts/DDS/DdsUnitTestsPlaceholder/ddsDataTopicTests_gtest.cpp

@@ -1,7 +1,16 @@
 #include <gtest/gtest.h>
+#include "mocks/MockDDS.h"
 
 TEST(DataTopicSmokeGTest, Format) {
   char buf[64];
   snprintf(buf, sizeof(buf), "%d-%s", 42, "abc");
   EXPECT_STREQ(buf, "42-abc");
 }
+
+TEST(DataTopicSmokeGTest, MockDDSTopicFlow) {
+  MockDDS::clear_all();
+  MockDDS::publish("data/topic", "hello");
+  EXPECT_EQ(MockDDS::take_last("data/topic"), "hello");
+  MockDDS::publish("data/topic", "world");
+  EXPECT_EQ(MockDDS::take_last("data/topic"), "world");
+}