(PUP-3605) use correct usermod flag on OpenBSD for group management

this allows 'membership => inclusive' to remove groups if needed as
'-G' only appends on OpenBSD and doesn't set.

The spec test is slightly unrelated in that it ensures the creation still uses `-G`, but currently there are no tests available for `objectadd.rb` (should be added, but it's out of scope for this PR).
Same as above, is this testing behavior implemented here or in the parent class? Actually, pretend I just said this a bunch of times. We should rely on the tests of the parent class to ensure correct behavior and only test what we override.