Neil Schemenauer: GC enable(), disable(), isenabled() interface.

Small stylistic changes by VM:
- is_enabled() -> isenabled()
- static ... Py_<func> -> static ... gc_<func>
diff --git a/Lib/test/test_gc.py b/Lib/test/test_gc.py
index 943f837..7f8e424 100644
--- a/Lib/test/test_gc.py
+++ b/Lib/test/test_gc.py
@@ -75,6 +75,11 @@
 
 
 def test_all():
+
+    enabled = gc.isenabled()
+    gc.disable()
+    assert not gc.isenabled()
+
     test_list()
     test_dict()
     test_tuple()
@@ -84,4 +89,11 @@
     test_finalizer()
     test_function()
 
+    # test gc.enable() even if GC is disabled by default
+    gc.enable()
+    assert gc.isenabled()
+    if not enabled:
+        gc.disable()
+
+
 test_all()