CMake tests that are build only when tests are run

CMakeLists.txt

cmake_minimum_required(VERSION 3.12 FATAL_ERROR)

project(MyProject)

add_executable(main src/main.cpp)

enable_testing()

# add a build target for the test executable
add_executable(testing src/testing.cpp)
# configure the target so it does not compile by default
set_target_properties(testing PROPERTIES EXCLUDE_FROM_ALL TRUE)

# add a test that actually compiles the text executable
# if this test fails, the test executable could not be compiled
# explicitly compile the target (`make testing` in this case) to see what failed
add_test(NAME TestExecutable COMMAND "${CMAKE_COMMAND}"
	--build "${CMAKE_BINARY_DIR}"
	--config "$<CONFIG>"
	--target "testing"
)

# add some test
add_test(Test1 testing)
# making sure it depends on the text executable, so it will be build
# before the test is run.
set_tests_properties(Test1 PROPERTIES DEPENDS TestExecutable)