tree: Make sure the set is always sorted

We use the __str__ to generate fingerprint and re-ordering the set leads
to different fingerprints.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/core/tree.py

@@ -59,7 +59,8 @@ class FilterSet(set):
                                               for item in items])
 
     def __str__(self):
-        return 'FilterSet([%s])' % ', '.join(["'%s'" % i for i in self])
+        return ('FilterSet([%s])'
+                % ', '.join(sorted(["'%s'" % i for i in self])))
 
 
 class TreeEnvironment(dict):