test_mux.TestMuxTree.test_basic_functions: get a string on Python 3

And not bytes.  What happens here is that
`avocado.core.tree.tree_view()` will return either bytes with 'utf-8'
or 'ascii' depending on the given parameter.

On Python 2, it's fine to compare the result with strings.  On Python
3, we have to get a string from the bytes returned, that is, we need
to decode the bytes.

While at it, let's fix a typo (s/asci/ascii).
Signed-off-by: NCleber Rosa <crosa@redhat.com>

optional_plugins/varianter_yaml_to_mux/tests/test_mux.py

@@ -127,9 +127,9 @@ class TestMuxTree(unittest.TestCase):
                   'prod']
         self.assertEqual(leaves, self.tree.get_leaves())
         # ascii contain all leaves and doesn't raise any exceptions
-        ascii = tree.tree_view(self.tree, 0, False)
+        ascii = tree.tree_view(self.tree, 0, False).decode('ascii')
         for leaf in leaves:
-            self.assertIn(leaf, ascii, "Leaf %s not in asci:\n%s"
+            self.assertIn(leaf, ascii, "Leaf %s not in ascii:\n%s"
                           % (leaf, ascii))
 
     def test_filters(self):