Fix CI: keep raw strings with backslashes out of test macros for MSVC

MSVC stringizes the arguments of doctest's CHECK() so that a raw string
literal becomes an ordinary one, and then reported the "\q" in the new
alt_string recovery test as warning C4129, an error with /WX. The input
is now a variable. The same pattern with "\u0000" in the parser's
recovery test is replaced by a JSON value built from a std::string.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-class_parser.cpp

@@ -3268,7 +3268,7 @@ TEST_CASE("parser error recovery (#3989)")
         const auto in_string = parse_recovering(std::string("[\"a\0b\"]", 7));
         CHECK(in_string.balanced);
 #ifdef JSON_TEST_STRICT_NUL_HANDLING_ENABLED
-        CHECK(in_string.value == json::parse(R"(["a\u0000b"])"));
+        CHECK(in_string.value == json::array({std::string("a\0b", 3)}));
 #else
         CHECK(in_string.value == json::parse(R"(["a"])"));
 #endif