fix(test/string): fix gcc 16 build issue

[ykshek]

This should fix a build issue for gcc16 and thus fedora 44, where the build fails due to test error:

/builddir/build/BUILD/dlib-20.0-build/dlib-20.0/dlib/test/string.cpp:148:59: 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"");`
      |                                                           ^~~
/builddir/build/BUILD/dlib-20.0-build/dlib-20.0/dlib/test/string.cpp:150:59: error: use of deleted function ‘std::basic_ostream<char, _Traits>& std::operator<<(basic_ostream<char, _Traits>&, const wchar_t*) [with _Traits = char_traits<char>]’
  150 |         DLIB_TEST_MSG( (right_substr(ws,L".") == L"txt"), L"");
      |                                                           ^~~

[ykshek]

https://en.cppreference.com/w/cpp/io/basic_ostream/operator_ltlt2.html