try to make bpf_existence_test saner

The core idea here is everything starts unexpected,
and then things are either removed from unexpected
and added to expected, or the reverse.

Test: TreeHugger, but not really, since too many combinations, hopefully any bugs will be found later...
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I44915e2ba459a8c90d9e706d46c6b89d85f0edd7