tests: fixup

tests/test_basic.py

@@ -85,7 +85,19 @@ def test_result_S_J0(omega, h, terminals):
     assert_allclose(J, 0, atol=1e-5)
 
 
-def test_gauge_invariance():
+@pytest.mark.parametrize(
+    "use_numdifftools",
+    [
+        False,
+        pytest.param(
+            True,
+            marks=pytest.mark.skipif(
+                numdifftools is None, reason="numdifftools not installed"
+            ),
+        ),
+    ],
+)
+def test_gauge_invariance(use_numdifftools):
     nx = 10
     ny = 7
     h = 0.4
@@ -179,7 +191,7 @@ def test_gauge_invariance():
     # They must match
     assert_allclose(div_J, dS_ds, rtol=1e-6)
 
-    if numdifftools is not None:
+    if use_numdifftools:
         # More accurate numdiff: the equality should hold to high
         # numerical accuracy
         D = numdifftools.Derivative(S_s)