cmake: build tests last

Keep -Werror for src, contrib and do not pass it for tests/

src/CMakeLists.txt

@@ -34,6 +34,11 @@ if (WIN32 OR STATIC)
   add_definitions(-DMINIUPNP_STATICLIB)
 endif ()
 
+# warnings are cleared only for GCC on Linux
+if (NOT (MINGW OR APPLE OR FREEBSD OR OPENBSD OR DRAGONFLY))
+  add_compile_options("${WARNINGS_AS_ERRORS_FLAG}") # applies only to targets that follow
+endif()
+
 function (monero_private_headers group)
   source_group("${group}\\Private"
     FILES