Fix [un]directed_graph::swap and add tests

[BenPope]

No description provided.

[BenPope]

Some other points you might like to comment on:

• adjacency_list::Base (detail::adj_list_gen) appears to be contain no members, but is copied and assigned, whilst maybe_named_graph appears to contain members, but is not copied or assigned.
• adjacency_list::swap looks suboptimal (copying, as it does).

Should I look into reimplementing adjacency_list::swap to be cheap and correct, and then reimplement copy and assignment in the idiomatic way?

I'm also considering free function swap, but that didn't seem to work when I gave it a quick try, perhaps because it should exist in the boost namespace (for ADL) and doesn't get picked up over boost/utility/swap, or I may have messed up the implementation.

[BenPope]

That was quick, nice one!