FunctionField._test_derivation: Do not test what cannot be tested

Author: Matthias Koeppe

src/sage/rings/function_field/function_field.py

@@ -745,7 +745,10 @@ def _test_derivation(self, **options):
         tester = self._tester(**options)
         S = tester.some_elements()
         K = self.constant_base_field().some_elements()
-        d = self.derivation()
+        try:
+            d = self.derivation()
+        except ImportError:
+            return
         from itertools import product
         # Non-zero
         tester.assertFalse(d.is_zero())