JenkinsRule#restart preserves the original root path

[strangelookingnerd]

My initial implementation of JenkinsRule#restart in #716 seems to be flawed in a sense that the underlying Jenkins instance is restored with a copy of the original JENKINS_HOME under a new path.

In jenkinsci/throttle-concurrent-builds-plugin#302 in became clear that this impacts some corner cases where this path is actually relevant for some plugins (see jenkinsci/throttle-concurrent-builds-plugin#302 (comment)).

This PR aims to remove this flaw. Now JenkinsRule#restart will start a new instance with a copy of the original JENKINS_HOME under the same path as the previous instance. This is achieved by creating a backup.zip from the current JENKINS_HOME before shutting it down and starting a new instance from the unzipped backup that has the same path as before.

I do not assume that this change will cause any incompatibilities as I can not find evidence of a tests validating / building upon the flaws of the initial implementation.

Ping @basil @jglick Your input would be very welcome.

Testing done

Adapted the existing tests.

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[jglick]

creating a backup.zip from the current JENKINS_HOME before shutting it down and starting a new instance from the unzipped backup

Sounds better, though still inferior to JenkinsSessionRule which does not need to touch files at all.