Add describe_endpoint and register_certificate_without_ca in iot_mock module with unittest (#3279)

Co-authored-by: Zhi Li <zhi_li@trendmicro.com>

moto/utilities/utils.py

@@ -0,0 +1,10 @@
+import random
+import string
+
+
+def random_string(length=None):
+    n = length or 20
+    random_str = "".join(
+        [random.choice(string.ascii_letters + string.digits) for i in range(n)]
+    )
+    return random_str