addPlugins does not currently take effect when used inside test method

[basil]

Moves the existing logic a little further down so that it takes effect in more scenarios.

[jglick]

I was considering this change earlier but rejected it because it is not exactly right, either. This copies specified files to $JENKINS_HOME/plugins/ before every startup, unlike the plugins from test-dependencies which are just copied once, when preparing $JENKINS_HOME initially.

Arguably what a caller would expect is for a call to addPlugins before the test runs to collect its arguments, including them in the initial list in apply; but a call after $JENKINS_HOME already exists to immediately copy the plugin there (once). A bit more complicated but seems better.

[jglick]

I suppose better would be for all the $JENKINS_HOME setup currently done eagerly inside apply should be deferred until startJenkins (or getHome) is first called.

[basil]

I have not found the time to get back to this, so closing this for now so we do not waste build resources on it. Great task for a volunteer to pick up!