Fix unistd.getpid_caching_and_clone.

This test only works if you're root (strictly: if you have permission to
CLONE_NEWNS), so it's useful to us when we're doing ad hoc testing (since
that's usually done as root), but it's not useful as part of CTS or when
running the tests on the host.

(cherry-pick of 84d0683a.)

Bug: 16705621
Bug: 17170200
Change-Id: Ia92c871b15f7e45fc174bb59bc95540fd00ae745