Print out line diff on test failure (#2552)

It currently prints both ASTs - this also
adds the line diff, making it much easier to visualize
the changes as well. Not too verbose since it's only a diff.

tests/util.py

@@ -9,7 +9,7 @@
 import black
 from black.debug import DebugVisitor
 from black.mode import TargetVersion
-from black.output import err, out
+from black.output import diff, err, out
 
 THIS_DIR = Path(__file__).parent
 DATA_DIR = THIS_DIR / "data"
@@ -47,6 +47,9 @@ def _assert_format_equal(expected: str, actual: str) -> None:
         except Exception as ve:
             err(str(ve))
 
+    if actual != expected:
+        out(diff(expected, actual, "expected", "actual"))
+
     assert actual == expected