Normalizes the Result rate to always float

Signed-off-by: NCaio Carrara <ccarrara@redhat.com>

avocado/core/result.py

@@ -114,6 +114,6 @@ class Result(object):
     def rate(self):
         total = float(self.tests_total - self.skipped - self.cancelled)
         if not total:
-            return 0
+            return 0.0
         succeeded = float(self.passed + self.warned)
         return 100 * (succeeded / total)