[FIXED JENKINS-4409] release class loaders for tests when JVM shutdown.

This is an improved fix for #788 .

When Jenkins stops, `PluginManager#stop` is called. It stops plugins and releases class loaders.
`TestPluginManager`, which is used when running tests, overrides stop and does not release class loaders. This is for `TestPluginManager` is re-used in every tests, and releasing class loaders in each test seems result in losing access to webapp directories of each plugin in sequential tests (`HelpLinkTest ` will fail).

On the other hand, not releasing class loaders result JVM not to release handles of jar files, and in Windows, temporary directories fail to be deleted when test finishes.

This patch releases class loaders as `PluginManager#stop` do when JVM shutsdown.