fix: only build tests if top level

CMakeLists.txt

@@ -3,7 +3,6 @@ project(curtle VERSION 0.1.1 LANGUAGES C)
 
 include(GNUInstallDirs)
 include(CMakePackageConfigHelpers)
-include(CTest)
 
 set(CMAKE_C_STANDARD 11)
 set(CMAKE_C_STANDARD_REQUIRED ON)
@@ -54,7 +53,7 @@ install(FILES
         DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/curtle
 )
 
-if(BUILD_TESTING)
+if(${PROJECT_IS_TOP_LEVEL})
     enable_testing()
     add_subdirectory(tests)
 endif()