fix(test/string): avoid use of deleted function

This should fix a build issue for gcc16+, where the build fails due to test `error: use of deleted function ‘std::basic_ostream<char, _Traits>& std::operator<<(basic_ostream<char, _Traits>&, const wchar_t*) [with _Traits = char_traits<char>]’
  148 |         DLIB_TEST_MSG( (left_substr(ws,L".") == L"file"), L"");`

Author: ykshek

dlib/test/string.cpp

@@ -145,9 +145,9 @@ namespace
 
         wstring ws = L"file.txt";
         DLIB_TEST( (left_substr(ws,wstring(L".")) == L"file"));
-        DLIB_TEST_MSG( (left_substr(ws,L".") == L"file"), L"");
+        DLIB_TEST_MSG( (left_substr(narrow(ws),L".") == L"file"), L"");
         DLIB_TEST( (right_substr(ws,wstring(L".")) == L"txt"));
-        DLIB_TEST_MSG( (right_substr(ws,L".") == L"txt"), L"");
+        DLIB_TEST_MSG( (right_substr(narrow(ws),L".") == L"txt"), L"");
 
 
         dlog << LTRACE << 8;