[FIXED JENKINS-4409] Use a fixed directory for TestPluginManager.

TestPluginManager expands plugins to a temporary directory at the start of all tests. This temporary directory is deleted with a shutdown hook of JVM. 

In Windows, for JVM holds the handle of jar files bundled with plugins, it always fails to delete the temporary directory where plugins expanded. That is, ${java.io.tmpdir}/hudson*tmp is left even after the test finishes. Every time a test runs, the size of the temporary directory grows up.

TestPluginManager seems not need to use a different directory every time. Using a fixed directory prevents the size of the temporary directory growing up.