avocado.core.tree: Add method to detach a node.

Add method to detach a node from the tree.
Signed-off-by: NRudá Moura <rmoura@redhat.com>

avocado/core/tree.py

@@ -192,6 +192,12 @@ class TreeNode(object):
         else:
             return [char1 + '-' + node_name], 0
 
+    def detach(self):
+        if self.parent:
+            self.parent.children.remove(self)
+            self.parent = None
+        return self
+
 
 def ordered_load(stream, Loader=yaml.Loader,
                  object_pairs_hook=collections.OrderedDict):

selftests/all/unit/avocado/tree_unittest.py

@@ -127,6 +127,11 @@ class TestTreeNode(unittest.TestCase):
         self.assertDictEqual(self.treenode.children[1].children[1].children[0].environment, {'dev-tools': 'cygwin', 'metro': False})
         self.assertDictEqual(self.treenode.children[1].children[1].children[1].environment, {'dev-tools': 'cygwin', 'metro': True})
 
+    def test_detach(self):
+        n = self.treenode.children[1].detach()
+        self.assertEqual(n.name, 'os')
+        self.assertNotIn(n, self.treenode.children)
+
 
 if __name__ == '__main__':
     unittest.main()