(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).
Is this testing behavior that's implemented in this provider or implemented in the parent class? If it's the latter we should omit these tests; if these tests fail then the parent tests will fail as well, and if these tests don't fail then the parent tests can't fail either.