[FIXED JENKINS-17774] Make not to copy a webapp directory every time for each tests.
Copying every time causes overwriting files in the webapp directory, and often causes errors in Windows.
This reverts JENKINS-12647(commit 7ae303f88191a540491a21484316d63dff775e21).
Merge pull request #779 from ikedam/feature/JENKINS-17774-2

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