Handle negative attempts and delay

If test author try to use attempts or delay with a value less than 1,
ValueError is raised.

checkbox-support/checkbox_support/helpers/retry.py

@@ -36,6 +36,16 @@ def run_with_retry(f, max_attempts, delay, *args, **kwargs):
     """
     initial_delay = 1
     backoff_factor = 2
+    if max_attempts < 1:
+        raise ValueError(
+            "max_attempts should be at least 1 ({} was used)".format(
+                max_attempts
+            )
+        )
+    if delay < 1:
+        raise ValueError(
+            "delay should be at least 1 ({} was used)".format(delay)
+        )
     for attempt in range(1, max_attempts + 1):
         attempt_string = "Attempt {}/{}".format(attempt, max_attempts)
         print()

checkbox-support/checkbox_support/tests/test_retry.py

@@ -53,6 +53,22 @@ def f():
         self.assertIn("Attempt 7 failed", mock_stdout.getvalue())
         self.assertNotIn("Attempt 8 failed", mock_stdout.getvalue())
 
+    def test_decorator_wrong_max_attempts(self):
+        @retry(-1, 10)
+        def f():
+            return 1 / 0
+
+        with self.assertRaises(ValueError):
+            f()
+
+    def test_decorator_wrong_delay(self):
+        @retry(2, -1)
+        def f():
+            return 1 / 0
+
+        with self.assertRaises(ValueError):
+            f()
+
     def test_identity(self):
         def k(*args, **kwargs):
             return (args, kwargs)