CMake: build tests only if not cross-compiling

CMakeLists.txt

@@ -31,4 +31,6 @@ set(PARSERS header dns dhcp http igmp ssdp coap)
 
 # Subdirectories containing code
 add_subdirectory(src)
-add_subdirectory(test)
+IF( NOT OPENWRT_CROSSCOMPILING )
+    add_subdirectory(test)
+ENDIF()