torture-krb5: Reformat and re-work test to be easier to follow

torture-krb5: Reformat and re-work test to be easier to follow

The behaviour is the same as in the previous commit, but it is much easier to follow
as the main test code now indicates to the send_and_recv callbacks what stage of the
test we are at, and resets the packet counter between stages.

This also re-orders the code so that the send and recv callbacks for each stage
are next to each other, and uses a case statement in the main send_and_recv driver
for clarity.

Andrew Bartlett

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Pair-programmed-with: Garming Sam <garming@catalyst.net.nz>
Signed-off-by: Garming Sam <garming@catalyst.net.nz>