[test] use cmake to run unit tests (#8724)

This commit migrates unit tests to cmake.
* build unit tests with cmake
* add missing tests in cmake
* use ctest
* add platform udp stubs for test platform
* skip some static_assert with gcc-4

CMakeLists.txt

@@ -33,6 +33,7 @@ file(READ .default-version OT_DEFAULT_VERSION)
 string(STRIP ${OT_DEFAULT_VERSION} OT_DEFAULT_VERSION)
 
 project(openthread VERSION ${OT_DEFAULT_VERSION})
+include(CTest)
 
 option(OT_BUILD_EXECUTABLES "Build executables" ON)
 option(OT_COVERAGE "enable coverage" OFF)
@@ -204,10 +205,6 @@ endif()
 add_subdirectory(src)
 add_subdirectory(third_party EXCLUDE_FROM_ALL)
 
-if(OT_PLATFORM STREQUAL "simulation")
-    enable_testing()
-endif()
-
 add_subdirectory(tests)
 add_subdirectory(tools)