[ROS-O] drop broken test hook from Fast-CDR import

[v4hn]

The tests were not imported but still referenced in the cmake depending on the system cmake version.

My build (Debian testing, ROS-O) just got stuck on:

CMake Error at plotjuggler_plugins/ParserROS/rosx_introspection/3rdparty/Fast-CDR/CMakeLists.txt:136 (add_subdirectory):
  add_subdirectory given source "test" which is not an existing directory.

It looks like you imported the library without its tests, but did not remove the cmake reference.