create agents with labels pre-set

[jtnord]

calling setLabels on an agent will not persist the node.

in older versions of Jenkins the tests would be less flaky as adding any
Node would cause all labels to be re-evaluated, so when creating a few
agents and adding labels in a loop the last one created would at least
deterministically ensure that all previous agents labels where correct.

However since 2.332 (jenkinsci/jenkins#5882)
only labels part of a node added or removed would be updated, and when
creating the agents they where created without labels, which where added
later.

This caused tests to be flaky depending on when the periodic
trimLabels was called (or at least on other timing related things)

Additionally creating agents and waiting for them to come oneline is
slow. A job can be scheduled and will then wait for the node to be available,
so we can do other things whilst the agent is connecting.

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