Adding test case for skip-magic-trailing-comma

Functionality added in pull request #48

tests/run_tests.sh

@@ -42,4 +42,9 @@ diff test_changes/hello_world_EOF.txt <(flake8 --select BLK test_changes/hello_w
 diff with_bad_toml/hello_world.txt <(flake8 --select BLK with_bad_toml/hello_world.py)
 diff with_pyproject_toml/ignoring_toml.txt <(flake8 with_pyproject_toml/ --select BLK --black-config '')
 
+# no changes by default,
+flake8 --select BLK test_changes/commas.py
+# will make changes if we ignore the magic trailing comma:
+diff test_changes/commas.txt <(flake8 --select BLK test_changes/commas.py --black-config with_pyproject_toml/pyproject.toml)
+
 echo "Tests passed."

tests/test_changes/commas.py

@@ -0,0 +1,24 @@
+"""Example of black and magic commas."""
+
+vegetables = {
+    "carrot",
+    "parsnip",
+    "potato",
+    "swede",
+    "leak",
+    "aubergine",
+    "tomato",
+    "peas",
+    "beans",
+}
+
+# This set would easily fit on one line, but a trailing comma
+# after the final entry tells black (by default) to leave this
+# with one entry per line:
+yucky = {
+    "aubergine",
+    "squid",
+    "snails",
+}
+
+print("I dislike these vegetables: %s." % ", ".join(vegetables.intersection(yucky)))

tests/with_pyproject_toml/pyproject.toml

@@ -1,2 +1,3 @@
 [tool.black]
 skip-string-normalization = true
+skip-magic-trailing-comma = true