fix: rename CLOCK_SKEW and separate client/server user case (#863)

* fix: rename CLOCK_SKEW and separate client/server user case

* update clock skew to 20s
diff --git a/tests/test_iam.py b/tests/test_iam.py
index 30ce227..e9eca58 100644
--- a/tests/test_iam.py
+++ b/tests/test_iam.py
@@ -45,7 +45,7 @@
             super(CredentialsImpl, self).__init__()
             self.token = "token"
             # Force refresh
-            self.expiry = datetime.datetime.min + _helpers.CLOCK_SKEW
+            self.expiry = datetime.datetime.min + _helpers.REFRESH_THRESHOLD
 
         def refresh(self, request):
             pass