Fixes JERSEY-2515 - after checking that it is possible to create files i...

...n java.io.tmpdir by attempting to create a file there, it should be deleted immediately
Sure!

I'm trying to find the right place to create the temporary directory and set java.io.tmp - it really just needs to be set for that particular test, but it has to be set early enough so that Jersey uses it. After trying a few things, I ended up having it as a static field and creating/deleting it with `@BeforeClass`/`@AfterClass`.