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_external_account.py b/tests/test_external_account.py
index e8297da..df6174f 100644
--- a/tests/test_external_account.py
+++ b/tests/test_external_account.py
@@ -976,7 +976,9 @@
         # Set the expiration to one second more than now plus the clock skew
         # accomodation. These credentials should be valid.
         credentials.expiry = (
-            datetime.datetime.min + _helpers.CLOCK_SKEW + datetime.timedelta(seconds=1)
+            datetime.datetime.min
+            + _helpers.REFRESH_THRESHOLD
+            + datetime.timedelta(seconds=1)
         )
 
         assert credentials.valid
@@ -1027,7 +1029,9 @@
         # Set the expiration to one second more than now plus the clock skew
         # accomodation. These credentials should be valid.
         credentials.expiry = (
-            datetime.datetime.min + _helpers.CLOCK_SKEW + datetime.timedelta(seconds=1)
+            datetime.datetime.min
+            + _helpers.REFRESH_THRESHOLD
+            + datetime.timedelta(seconds=1)
         )
 
         assert credentials.valid