Convert test_optimize_correctness to use parametrize

Tests/test_file_gif.py

@@ -158,12 +158,28 @@ def test_optimize():
     assert test_bilevel(1) == 799
 
 
-def test_optimize_correctness():
-    # 256 color Palette image, posterize to > 128 and < 128 levels
-    # Size bigger and smaller than 512x512
+@pytest.mark.parametrize(
+    "colors, size, expected_palette_length",
+    (
+        # These do optimize the palette
+        (256, 511, 256),
+        (255, 511, 255),
+        (129, 511, 129),
+        (128, 511, 128),
+        (64, 511, 64),
+        (4, 511, 4),
+        # These don't optimize the palette
+        (128, 513, 256),
+        (64, 513, 256),
+        (4, 513, 256),
+    ),
+)
+def test_optimize_correctness(colors, size, expected_palette_length):
+    # 256 color Palette image, posterize to > 128 and < 128 levels.
+    # Size bigger and smaller than 512x512.
     # Check the palette for number of colors allocated.
-    # Check for correctness after conversion back to RGB
-    def check(colors, size, expected_palette_length):
+    # Check for correctness after conversion back to RGB.
+
     # make an image with empty colors in the start of the palette range
     im = Image.frombytes(
         "P", (colors, colors), bytes(range(256 - colors, 256)) * colors
@@ -179,19 +195,6 @@ def test_optimize_correctness():
 
         assert_image_equal(im.convert("RGB"), reloaded.convert("RGB"))
 
-    # These do optimize the palette
-    check(256, 511, 256)
-    check(255, 511, 255)
-    check(129, 511, 129)
-    check(128, 511, 128)
-    check(64, 511, 64)
-    check(4, 511, 4)
-
-    # These don't optimize the palette
-    check(128, 513, 256)
-    check(64, 513, 256)
-    check(4, 513, 256)
-
 
 def test_optimize_full_l():
     im = Image.frombytes("L", (16, 16), bytes(range(256)))