Fix sys_prctl.pr_cap_ambient.

This test didn't clear errno before testing it, so was sensitive to
whatever code had run before. Since I'd already added the code to dump
the exact kernel version before realizing the actual bug, I've left that
in too.

Bug: http://b/121156651
Test: ran tests
Change-Id: I38482036cb6f035c0c98c27eb64d17829f29b09a