Remove killing xvfb since it can be shared.

WIP do not QA/review/land

Removing killing xvfb so that test and merge jobs may run side by side.

- Currently if a test and a merge (landing) job are running on Jenkins they interfere and will not pass due to no fault of the dev.
- This is due to one of them killing the xvfb process running while the other is sharing it.
- After a pre-imp with Benji it was decided there's no harm in leaving it running currently and it's the most expedient way to getting a safer CI process into place. 

Notes:

We may want to revisit this, as noted in the docs in line, if we want to do things like snapshot images in case of a failing test, etc. We can then make the xvfb port configurable as part of that work.