seccomp: Adjust selftests to avoid double-join

While glibc's pthread implementation is rather forgiving about repeat
thread joining, Bionic has recently become much more strict. To deal with
this, actually track which threads have been successfully joined and kill
the rest at teardown.

Based on a patch from Paul Lawrence.

Cc: Paul Lawrence <paullawrence@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>