580782: EPub unit tests are platform-agnostic

Fix a few issues with the epub unit tests that led to failed asserts
on a Windows host.

* No longer assumes the *Nix specific file path character.
* Uses the JUnit TemporaryFolder rule for better isolation between
tests. A failure to delete files during test tear down (e.g. due to
windows file locking) won't cause subsequent tests to fail.

Change-Id: I3dd383d950ff6a6f82d19ab727145086c8080d87
Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=580782