Don't send EVT_TEXT_ENTER to controls without wxTE_PROCESS_ENTER

wxMSW always sent this event to multiline text controls, even when they
didn't have wxTE_PROCESS_ENTER style, contrary to what was documented.

Avoid sending this event unless wxTE_PROCESS_ENTER is used and add unit
tests checking that multiline text controls don't get it without this
style (but still do get it with it).

src/msw/textctrl.cpp

@@ -2075,14 +2075,18 @@ void wxTextCtrl::OnChar(wxKeyEvent& event)
     switch ( event.GetKeyCode() )
     {
         case WXK_RETURN:
+            // Single line controls only get this key code if they have
+            // wxTE_PROCESS_ENTER style, but multiline ones always get it
+            // because they need it for themselves. However we shouldn't
+            // generate wxEVT_TEXT_ENTER for the controls without this style,
+            // so test for it explicitly.
+            if ( HasFlag(wxTE_PROCESS_ENTER) )
             {
                 wxCommandEvent evt(wxEVT_TEXT_ENTER, m_windowId);
                 InitCommandEvent(evt);
                 evt.SetString(GetValue());
                 if ( HandleWindowEvent(evt) )
-                    if ( !HasFlag(wxTE_MULTILINE) )
-                        return;
-                    //else: multiline controls need Enter for themselves
+                    return;
             }
             break;