Fix global UDLs (#3646)

* Add ci_test_noglobaludls to CI

* Really default JSON_GLOBAL_UDLS to 1

* Test global UDLs

* Suppress warnings

* Clarify documentation

tests/src/unit-class_parser.cpp

@@ -11,7 +11,9 @@
 #define JSON_TESTS_PRIVATE
 #include <nlohmann/json.hpp>
 using nlohmann::json;
-using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#ifdef JSON_TEST_NO_GLOBAL_UDLS
+    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#endif
 
 #include <valarray>