cbuildbot_stages_unittest: fix up unit test
cbuildbot_stages_unittest: fix up unit test

This fixes a failure introduced by the recent:
	cros_mark_as_stable: regenerate cache for portage-stable tree

BUG=chromium-os:30916
TEST=`./cbuildbot_stages_unittest.py` passes

Change-Id: I3229f88431358a80d7bcd99ca64f2459009d6ebe
Reviewed-on: https://gerrit.chromium.org/gerrit/22717
Reviewed-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

zhang0137/chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,coreos/chromite,zhang0137/chromite,zhang0137/chromite,coreos/chromite,chadversary/chromiumos.chromite,chadversary/chromiumos.chromite