tests/cmdline/cmd_showbc.py: Fix test to explicitly declare nonlocal.

The way it was written previously the variable x was not an implicit
nonlocal, it was just a normal local (but the compiler has a bug which
incorrectly makes it a nonlocal).

tests/cmdline/cmd_showbc.py

@@ -108,7 +108,7 @@ def f():
     # closed over variables
     x = 1
     def closure():
-        a = x + 1
+        nonlocal x; a = x + 1
         x = 1
         del x