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
Can you, please, move this test into `MultiPartReaderWriterTest` and call

    // Make sure the MBR is initialized on the client-side as well.
    target().request().get();

before asserting the temp directory has no files in it?