Add unittests to PRESUBMIT

BUG=None
TEST=repo upload runs unittests.

Change-Id: Ia1882a649f0ba81102ecc05a73c16d2dc4da74e7
Reviewed-on: https://chromium-review.googlesource.com/1178307
Commit-Ready: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Xixuan Wu <xixuan@chromium.org>
diff --git a/PRESUBMIT.cfg b/PRESUBMIT.cfg
index b05e81e..b214a43 100644
--- a/PRESUBMIT.cfg
+++ b/PRESUBMIT.cfg
@@ -1,3 +1,4 @@
 [Hook Scripts]
 cros lint = ../../chromite/bin/cros lint ${PRESUBMIT_FILES}
 check_proto_bindings = ./bin/check_proto_bindings
+unittests = ./bin/test_venv_packages