[JENKINS-23770] Making JUT server (PooledJenkinsController) work again.
c9b925d (@stephenc) introduced a populateJenkinsHome but only partly implemented it, in particular failing to implement in PooledJenkinsController and ExistingJenkinsController.
c490ff8 (@ogondza) then made this mandatory for all tests.
a62ba10 (@kohsuke) fixed one fatal problem with c490ff8 (by packing tool_installers.zip) but did not address this one.
Not marking FIXED since this change does not yet address ExistingJenkinsController.