test(auth): fix flaky assertion in test_external_accounts.py

Author: macastelaz

packages/google-auth/system_tests/system_tests_sync/test_external_accounts.py

@@ -216,8 +216,13 @@ def check_impersonation_expiration():
 
         credentials.refresh(http_request)
 
-        utcmax = _helpers.utcnow() + datetime.timedelta(seconds=TOKEN_LIFETIME_SECONDS)
-        utcmin = utcmax - datetime.timedelta(seconds=BUFFER_SECONDS)
+        now = _helpers.utcnow()
+        utcmax = now + datetime.timedelta(
+            seconds=TOKEN_LIFETIME_SECONDS + BUFFER_SECONDS
+        )
+        utcmin = now + datetime.timedelta(
+            seconds=TOKEN_LIFETIME_SECONDS - BUFFER_SECONDS
+        )
         assert utcmin < credentials._impersonated_credentials.expiry <= utcmax
 
         return True