tests: Add small testcase for 3-arg slices.

tests/basics/list_slice_3arg.py

@@ -2,3 +2,8 @@ x = list(range(10))
 print(x[::-1])
 print(x[::2])
 print(x[::-2])
+
+x = list(range(9))
+print(x[::-1])
+print(x[::2])
+print(x[::-2])