Don't corrupt UNC paths when normalizing with wxPATH_NORM_LONG

After the fixes to UNC paths handling, Normalize(wxPATH_NORM_LONG)
started adding an extra colon after the UNC paths.

Fix this by using wxGetVolumeString() instead of constructing this thing
manually and add a unit test checking for this.

Closes #22275.

src/common/filename.cpp

@@ -2240,8 +2240,7 @@ wxString wxFileName::GetLongPath() const
     HANDLE hFind;
 
     if ( HasVolume() )
-        pathOut = GetVolume() +
-                  GetVolumeSeparator(wxPATH_DOS) +
+        pathOut = wxGetVolumeString(GetVolume(), wxPATH_DOS) +
                   GetPathSeparator(wxPATH_DOS);
     else
         pathOut.clear();

tests/filename/filenametest.cpp

@@ -561,6 +561,14 @@ TEST_CASE("wxFileName::UNC", "[filename]")
     CHECK( fn.GetVolume() == "\\\\share2" );
     CHECK( GetDOSPath(fn) == "\\path2" );
 
+    fn.SetPath("\\\\server\\volume\\path", wxPATH_DOS);
+    fn.AppendDir("subdir");
+    CHECK( fn.GetFullPath(wxPATH_DOS) == "\\\\server\\volume\\path\\subdir\\name.ext" );
+
+    // Check for a bug with normalization breaking the path (#22275).
+    fn.Normalize(wxPATH_NORM_LONG);
+    CHECK( fn.GetFullPath(wxPATH_DOS) == "\\\\server\\volume\\path\\subdir\\name.ext" );
+
 #ifdef __WINDOWS__
     // Check that doubled backslashes in the beginning of the path are not
     // misinterpreted as UNC volume when we have a drive letter in the