Fix clang-tidy raw-string-literal finding and guard a new test against JSON_NOEXCEPTION

The issue #5412 whitespace-skipping test added a check_error() helper that
relies on catching json::parse_error to verify the exception message; under
JSON_NOEXCEPTION, JSON_THROW aborts instead of throwing, which crashed
ci_test_noexceptions (and cascaded into the other ci_cmake_options jobs).
Guard the whole section with #if !defined(JSON_NOEXCEPTION), matching the
existing pattern used by sibling tests in this file.

Also switch one escaped string literal to a raw string literal to satisfy
clang-tidy's modernize-raw-string-literal check.

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

tests/src/unit-class_parser.cpp

@@ -1486,6 +1486,7 @@ TEST_CASE("parser class")
         CHECK(accept_helper("\"\\uD80C\\uFFFF\"") == false);
     }
 
+#if !defined(JSON_NOEXCEPTION)
     SECTION("issue #5412 - whitespace skipping bookkeeping (compact vs. pretty-printed)")
     {
         // lexer::skip_whitespace() reads its first character with get() (to
@@ -1545,9 +1546,10 @@ TEST_CASE("parser class")
     "c": @
 })", 70,
                     "[json.exception.parse_error.101] parse error at line 7, column 10: syntax error while parsing value - invalid literal; last read: '\"c\": @'");
-        check_error("{\"a\":1,\"b\":[true,false],\"c\":@}", 29,
+        check_error(R"({"a":1,"b":[true,false],"c":@})", 29,
                     "[json.exception.parse_error.101] parse error at line 1, column 29: syntax error while parsing value - invalid literal; last read: '\"c\":@'");
     }
+#endif
 
     SECTION("tests found by mutate++")
     {