This makes the tests a little bit more self-documenting, and makes it
simpler to add new tests. Also, change it to only look at OVS flows in
the egress table, so that when there is a mismatch, the error message
is of a manageable length.

Split out one test case, add some new (already-passing) test cases,
add some comments, and remove some double-checking-of-internal-state
that will break with upcoming rewrites.

…ng one

(We handled delete+recreate correctly, but not update.)

There should never be multiple HostSubnets or multiple NetNamespaces
claiming the same egress IP, but if there are, we need to track them
carefully so we don't get out sync with reality after things are
fixed.