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_credentials.py b/tests/test_credentials.py
index 0633b38..2de6388 100644
--- a/tests/test_credentials.py
+++ b/tests/test_credentials.py
@@ -46,7 +46,9 @@
     # Set the expiration to one second more than now plus the clock skew
     # accomodation. These credentials should be valid.
     credentials.expiry = (
-        datetime.datetime.utcnow() + _helpers.CLOCK_SKEW + datetime.timedelta(seconds=1)
+        datetime.datetime.utcnow()
+        + _helpers.REFRESH_THRESHOLD
+        + datetime.timedelta(seconds=1)
     )
 
     assert credentials.valid