[JENKINS-17774] Plugin tests often fails in Windows.

For details, please refer to [JENKINS-17774] (https://issues.jenkins-ci.org/browse/JENKINS-17774).

Plugin tests using WebClient often fails in Windows 8 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] (https://issues.jenkins-ci.org/browse/JENKINS-12647) and 7ae303f88191a540491a21484316d63dff775e21.
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.

This patch does followings to HudsonTestCase:
* reverts JENKINS-12467.
* Extract createWebAppContext from createWebServer. This makes it easy for developers to customize WebAppContext if needed.