Adding test case for black preview setting

Functionality added in pull request #48, test case
suggested by @ferdynice

tests/run_tests.sh

@@ -43,8 +43,10 @@ diff with_bad_toml/hello_world.txt <(flake8 --select BLK with_bad_toml/hello_wor
 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
+flake8 --select BLK test_changes/commas.py tests/black_preview.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)
+# will make changes if we enable future functionality preview mode:
+diff test_changes/black_preview.txt <(flake8 --select BLK test_changes/black_preview.py --black-config with_pyproject_toml/pyproject.toml)
 
 echo "Tests passed."

tests/test_changes/black_preview.py

@@ -0,0 +1,11 @@
+"""Example showing future black string reformatting."""
+
+
+def hello():
+    """Print variations on 'Hello World'."""
+    # black v22.1.0 (first stable release) does not edit string literals,
+    # so the following is untouched unless enable preview mode:
+    print("hello " "world")  # noqa: ISC001
+
+
+hello()

tests/test_changes/black_preview.txt

@@ -0,0 +1 @@
+test_changes/black_preview.py:8:18: BLK100 Black would make changes.

tests/with_pyproject_toml/pyproject.toml

@@ -1,3 +1,4 @@
 [tool.black]
 skip-string-normalization = true
 skip-magic-trailing-comma = true
+preview = true