run_tests: run cros_list_modified_packages_unittest.py inside the chroot
run_tests: run cros_list_modified_packages_unittest.py inside the chroot

This script tries to import portage modules, so we need to run it inside
the chroot for it to work.

BUG=chromium:357653
TEST=`./buildbot/run_tests` passes outside the chroot

Change-Id: I39ff9fcd507f89fce78f7c50113444bd24f56212
Reviewed-on: https://chromium-review.googlesource.com/200049
Reviewed-by: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

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