fix(test/string): fix gcc 16 build issue (#3137)

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

Remove portion of code to fix a build issue for gcc16(C++23), 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"");`

* fix(test/tokenizer): remove u8"..."

C++20 fails due to convert from ‘<brace-enclosed initializer list>’ to ‘std::vector<std::__cxx11::basic_string<char> >’

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

This reverts commit ad56f13.

* fix(test/string): Attempt fix again

without deleting the test

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(ws,L".") == L"file"), "");
         DLIB_TEST( (right_substr(ws,wstring(L".")) == L"txt"));
-        DLIB_TEST_MSG( (right_substr(ws,L".") == L"txt"), L"");
+        DLIB_TEST_MSG( (right_substr(ws,L".") == L"txt"), "");
 
 
         dlog << LTRACE << 8;

dlib/test/tokenizer.cpp

@@ -394,10 +394,10 @@ namespace
         deserialize(loaded_test, in_stream);
 
         std::vector<std::string> test_strings = {
-            u8"This is a test of the tokenisation process...\nimplemented in the Dlib library!", // English
-            u8"Ceci est un test du processus de\ntokenisation implémenté dans\nla bibliothèque Dlib!", // French
-            u8"Dette er en test af tokeniseringsprocessen implementeret i Dlib-biblioteket!", // Danish
-            u8"这是对Dlib库中实现的标记化过程的测试！" // Chinese
+            "This is a test of the tokenisation process...\nimplemented in the Dlib library!", // English
+            "Ceci est un test du processus de\ntokenisation implémenté dans\nla bibliothèque Dlib!", // French
+            "Dette er en test af tokeniseringsprocessen implementeret i Dlib-biblioteket!", // Danish
+            "这是对Dlib库中实现的标记化过程的测试！" // Chinese
         };
 
         for (const auto& text : test_strings) {