Fix MSVC C2220 in the std::u8string conversion test

MSVC's C5321 ("nonstandard extension used: encoding '\xNN' as a
multi-byte utf-8 character") is promoted to a hard error by our MSVC CI
configs. It fires because the test composed a non-ASCII UTF-8 sequence
inside a u8"" literal using raw \x byte escapes; MSVC treats that as
nonstandard and suggests using \u universal-character-names instead,
which every compiler agrees on and which compiles down to the exact
same encoded bytes.

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

tests/src/unit-conversions.cpp

@@ -1779,7 +1779,14 @@ TEST_CASE("std::u8string")
 
     SECTION("utf-8")
     {
-        const std::u8string s = u8"P\xc4\x9b\xc5\xa1ina";
+        // use \u universal-character-names (rather than raw \x byte escapes
+        // or literal non-ASCII source bytes) to compose the multi-byte UTF-8
+        // encoding -- MSVC treats \x escapes used that way inside a u8
+        // literal as a nonstandard extension (warning C5321), which some of
+        // our CI configs promote to an error; \u is portable and produces
+        // the exact same encoded bytes without depending on the source
+        // file's encoding
+        const std::u8string s = u8"P\u011B\u0161ina";
         json const j = s;
 
         CHECK(j.template get<std::string>() == "P\xc4\x9b\xc5\xa1ina");