TST: Parametrize numpy roundtrip to find failing case

Segfaults are annoying to debug.

.github/workflows/test-cygwin.yml

@@ -38,6 +38,7 @@ jobs:
         run: |
           C:\tools\cygwin\bin\python3.8 -m pip install pip wheel setuptools
           C:\tools\cygwin\bin\python3.8 -m pip install -r requirements.txt
+          C:\tools\cygwin\bin\python3.8 -m pip install pytest-forked
 
       - name: Build Pillow
         run: |
@@ -56,6 +57,7 @@ jobs:
           C:\tools\cygwin\bin\mv .ci/test.sh .ci/test.sh.dos
           C:\tools\cygwin\bin\dash -c "/bin/tr -d '\r' <.ci/test.sh.dos >.ci/test.sh"
           C:\tools\cygwin\bin\chmod u+x .ci/test.sh
+          C:\tools\cygwin\bin\python3.8 -m pytest --forked Tests/test_numpy.py -k roundtrip
           C:\tools\cygwin\bin\dash /usr/bin/xvfb-run -s '-screen 0 1024x768x24' .ci/test.sh .8
 
       - name: After success

Tests/test_numpy.py

@@ -189,8 +189,9 @@ def test_putdata():
     assert len(im.getdata()) == len(arr)
 
 
-def test_roundtrip_eye():
-    for dtype in (
+@pytest.mark.parametrize(
+    "dtype",
+    (
         bool,
         numpy.bool8,
         numpy.int8,
@@ -202,9 +203,11 @@ def test_roundtrip_eye():
         float,
         numpy.float32,
         numpy.float64,
-    ):
-        arr = numpy.eye(10, dtype=dtype)
-        numpy.testing.assert_array_equal(arr, numpy.array(Image.fromarray(arr)))
+    ),
+)
+def test_roundtrip_eye(dtype):
+    arr = numpy.eye(10, dtype=dtype)
+    numpy.testing.assert_array_equal(arr, numpy.array(Image.fromarray(arr)))
 
 
 def test_zero_size():