Remove ensure_dirs double-patch

Remove ensure_dirs double-patch

test_spawn_radvd called mock.patch on ensure_dirs after the
setup method already patched it out. This causes issues when
mock.patch.stopall() is called because the mocks are stored
as a set and are unwound in a non-deterministic fashion.[1]
So some of the time they will be undone correctly, but others
will leave a monkey-patched in mock, causing the ensure_dir
test to fail.

1. http://bugs.python.org/issue21239

Closes-Bug: #1467908
Change-Id: I321b5fed71dc73bd19b5099311c6f43640726cd4