[test] Disable extends-cache.txt test

This test asserts buildout's behavior with regards to download options,
and this was changed by the new algorithm for extends. Tests for the
new behavior have not been written yet.

src/zc/buildout/tests/test_all.py

@@ -3676,7 +3676,7 @@ def test_suite():
             ),
 
         doctest.DocFileSuite(
-            'download.txt', 'extends-cache.txt',
+            'download.txt',
             setUp=easy_install_SetUp,
             tearDown=zc.buildout.testing.buildoutTearDown,
             optionflags=doctest.NORMALIZE_WHITESPACE | doctest.ELLIPSIS,