Add a simple acceptance test for "submit whole topic"

Add a test that uploads two changes on the same topic, then
submits the second one and asserts that both changes have been
merged.

The following cases are not covered and need to be added in
follow-up changes:

- Changes on different projects and branches with the same topic
- Changes with the same topic that are not submittable

Change-Id: I2323caf1bb56232f4c834a48b51729cac8db7a05
2 files changed