[JENKINS-36177] Acceptance test for Multi-Branch flow

Create a v1 acceptance test for testing the the multibranch flow:

Create the multibranch job. Will need to have branches.
Inspect structure of the Branches tab. Use a Nightwatch page-object for this.
Setup an SSE listener in the test to listen for runs of one of the branches.
Press the "run" widget on the branch being listened to.
Use the async SSE events to move the test to a successful "done()".
Verify that the "Queued" and "Started" Toast components appeared at the right time. This will be tricky hmmm.

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