tests: remove firewalld backend tests from virfirewalltest.c

tests: remove firewalld backend tests from virfirewalltest.c

When libvirt added support for firewalld, all iptables/ebtables rules
were added via the firewalld "passthrough" API when firewalld was
enabled (the "firewalld backend"), or run directly by libvirt when
firewalld was disabled (the so-called "direct
backend"). virfirewalltest.c dutifully ran each test twice, once with
the each backend enabled.

But commit b19863640d changed the code to *always* directly run
iptables/ebtables commands, and never use the firewalld passthrough
API, effectively making the direct and firewalld backends identical,
except that when libvirt receives notice that firewalld has restarted
or reloaded its rules, the firewalld backend sends an extra "iptables
-V" command via firewalld's passthrough API (and waits for a response)
prior to running all the rest of the iptables commands directly; this
assures that a newly-restarted firewalld has finished its work on the
filter tables before libvirt starts messing with it. (Because this
code is only executed in response to an event from dbus, it isn't
tested in the unit tests).

In spite of this, we still go through all the virfirewall tests twice
though - once for the direct backend, and once for the firewalld
backend, even though these take the same codepath.

In commit b19863640d I had left this double-testing in thinking that
someday we might go back to actually doing something useful with the
firewalld backend in the course of adding support for native nftables,
but I've now realized that for the case of nftables we will be *even
more* divorced from firewalld, so there is really no point in keeping
this code around any longer. (It's likely/probable that the tests will
be done twice again in the future, but it will be enough different
that it is better to remove this code and re-implement from scratch
when adding the nftables backend, rather than trying to directly
modify the existing code and end up with something even more
confusing).

This patch eliminates all the test duplication in virfirewalltest.c,
including mocking dbus, which is unnecessary since none of the tests
use dbus (for now we ensure that by explicitly setting the virfirewall
backend to DIRECT before any of the tests have run. Eventually the
concept of a "firewalld backend" will disappear completely, but that's
for another patch.)

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>