Remove killing xvfb since it can be shared.
Merge pull request #42 from mitechie/test-merge-singularity

- 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.

QA
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Note that below we have failed test and merge runs. I've updated the branch to remove the failing test and it's ready for review.