Enable ctest with gtest and some basic unit tests for CI

This series enables ctest together with gtest as our unit test framework, adds some basic test cases for verify its functionality and hooks it up into CI.

tests/CMakeLists.txt

+add_executable(
+  communication_tests
+  communication_tests.cpp
+)
+target_link_libraries(
+  communication_tests
+  gtest
+  gtest_main
+  PkgConfig::Coap
+  eddie-communication
+)
+
+include(GoogleTest)
+gtest_discover_tests(communication_tests)