build: format-check: invoke make in a portable way

I've added `uncrustify` to my build environment, and as a result the
build started failing for me because it tried to invoke `make
format-check`. I use the ninja generator for CMake, which means that
there's no `Makefile` generated in my build directory.

Fix this by invoking the relevant target via CMake.

Author: jktjkt

tests/CMakeLists.txt

@@ -4,7 +4,7 @@ add_test(NAME headers
 
 # format
 if (${SOURCE_FORMAT_ENABLED})
-    add_test(NAME format WORKING_DIRECTORY ${CMAKE_BINARY_DIR} COMMAND make format-check)
+    add_test(NAME format WORKING_DIRECTORY ${CMAKE_BINARY_DIR} COMMAND cmake --build ${CMAKE_BINARY_DIR} --target format-check)
 endif()
 
 # list of all the tests in each directory