selftests: unshare userns in seccomp pidns testcases

The pid ns cannot be unshare()d as an unprivileged user without owning the
userns as well. Let's unshare the userns so that we can subsequently
unshare the pidns.

This also means that we don't need to set the no new privs bit as in the
other test cases, since we're unsharing the userns.

Signed-off-by: Tycho Andersen <tycho@tycho.ws>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <shuah@kernel.org>