[FIXED JENKINS-17774] Plugin tests often fails in Windows (just reverting JENKINS-12647).

For details, please refer to [JENKINS-17774] (https://issues.jenkins-ci.org/browse/JENKINS-17774).
This is an alternate pull request to #773 (already abandoned by myself).

Plugin tests using WebClient often fails in Windows8 with TortoiseGit installed.
Those failures are caused by Jetty failing to overwrite jenkins-for-test onto a webapps directory, which is caused for Windows often stay holding file handles.

This Jetty overwriting behavior in Windows is introduced in JENKINS-12467 and 7ae303f.
But as far as I tested, the problem reported on JENKINS-12467 is not caused by Jetty behavior, so changes in JENKINS-12467 can be reverted.
And this behavior is only with `HudsonTestCase`, but not with `JenkinsRule`.

This patch reverts JENKINS-12467.