Favor network identities in erc-join

Favor network identities in erc-join

* lisp/erc/erc-join.el (erc-autojoin-server-match): Favor network
identities, falling back on old definition.
(erc-autojoin--join): Add new helper containing common code from
hookees `erc-autojoin-after-ident' and `erc-autojoin-channels'.
(erc-autojoin-after-ident, erc-autojoin-channels): No longer make a
point of returning nil because the hooks they're registered on,
`erc-nickserv-identified-hook' and `erc-after-connect', don't stop on
success.
(erc-autojoin--mutate): Add helper for `erc-autojoin-add' and
`erc-autojoin-remove'.
(erc-autojoin-add, erc-autojoin-remove): Favor given network
identities, over networks, when matching keys for
`erc-autojoin-channels-alist'.

* test/lisp/erc/erc-scenarios-base-reconnect.el: New file.
* test/lisp/erc/erc-scenarios-join-netid-newcmd-id.el: New file.
* test/lisp/erc/erc-scenarios-join-netid-newcmd.el: New file.
* test/lisp/erc/erc-scenarios-join-netid-recon-id.el: New file.
* test/lisp/erc/erc-scenarios-join-netid-recon.el: New file.
* test/lisp/erc/resources/erc-scenarios-common.el: New file.
* test/lisp/erc/resources/join/legacy/foonet.eld: New file.
* test/lisp/erc/resources/join/network-id/barnet.eld: New file.
* test/lisp/erc/resources/join/network-id/foonet-again.eld: New file.
* test/lisp/erc/resources/join/network-id/foonet.eld: New file.
* test/lisp/erc/resources/join/reconnect/foonet-again.eld: New file.
* test/lisp/erc/resources/join/reconnect/foonet.eld: New file.