Fix bulk's unit tests on Mac OS.

Turns out if your $TMPDIR is really long
(e.g. /var/folders/vq/n32yszdn4s76z6l8dxklmwdh0000gn/T/, which is 50
characters), then the test that drops stuff into $TMPDIR and adds it
to a tarball and uploads it will fail due to every added file's name
being too long.

Change-Id: I8fcab2d95091f72b831b906bfc87a36d8be12631
Merge "Fix bulk's unit tests on Mac OS."