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-alt-string.cpp

@@ -395,7 +395,9 @@ TEST_CASE("alternative string type")
 
         alt_json j;
         recovering_parser sax(j);
-        CHECK(!alt_json::sax_parse(R"([1., "a\qb", tru, {"k" 2}])", &sax));
+        // not inside CHECK(): MSVC reads the escape in a stringized raw string
+        const std::string input = R"([1., "a\qb", tru, {"k" 2}])";
+        CHECK(!alt_json::sax_parse(input, &sax));
         CHECK(sax.errors == 4);
         CHECK(j.dump() == R"([1,"aqb",null,{"k":2}])");