avocado.core.tree: Show multiplex in tree view

Multiplexed nodes affects heavily the tree structure and users should
see i in the tree view. This patch makes "multiplex" nodes visible.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/core/tree.py

@@ -292,6 +292,8 @@ class TreeNode(object):
         node_name = ', '.join(map(str, [getattr(self, v)
                                         for v in attributes
                                         if hasattr(self, v)]))
+        if self.multiplex:
+            node_name += "-<>"
 
         length = max(2, (len(node_name) + 1) if not self.children or show_internal else 3)
         pad = ' ' * length