[#58637350] Wrapper script for branch testing

Calls `./jenkins.sh` as before and submits results back to GitHub status
API so that we get the pretties on pull requests.
[#58637350] Don't use set -u in wrapper script

It seems that virtualenv doesn't like this. Sad face.

Installing pip...............done.
.venv/bin/activate: line 6: _OLD_VIRTUAL_PATH: unbound variable
Build step 'Execute shell' marked build as failure
Finished: FAILURE
Merge pull request #20 from alphagov/58637350-branch_testing

[Delivers #58637350] CI branch testing