add extra warnings for tests

untests/CMakeLists.txt

@@ -29,4 +29,11 @@ add_executable(${PROJECT_NAME} ${UNTESTS_SOURCES})
 target_link_libraries(${PROJECT_NAME}
     Catch2
     flat.hpp)
+target_compile_options(${PROJECT_NAME}
+    PRIVATE
+    $<$<CXX_COMPILER_ID:MSVC>:
+        /W4>
+    PRIVATE
+    $<$<OR:$<CXX_COMPILER_ID:GNU>,$<CXX_COMPILER_ID:Clang>,$<CXX_COMPILER_ID:AppleClang>>:
+        -Wall -Wextra -Wpedantic>)
 add_test(${PROJECT_NAME} ${PROJECT_NAME})