WIP: gdbus-server-auth: Don't usually test non-EXTERNAL repeatedly

Even after changing the timeouts for #2164 (closed), DBUS_COOKIE_SHA1 doesn't seem to be 100% reliable. EXTERNAL is what we actually hope to be using on practical Unix systems (notably Linux and several flavours of BSD), so let's test that thoroughly by default, and just do a relatively cursory check on DBUS_COOKIE_SHA1 and ANONYMOUS.

In particular, the original purpose of this test was to make sure that EXTERNAL authentication over AF_UNIX sockets interoperated with libdbus reliably; it only tests DBUS_COOKIE_SHA1, ANONYMOUS and TCP out of a sense of completeness.