Set erc-network to a "given" ID instead of failing

Set erc-network to a "given" ID instead of failing

* lisp/erc/erc-networks.el (erc-networks--determine): Return the
so-called "given" ID from a non-nil `:id' keyword arg passed to `erc'
or `erc-tls'.
(erc-networks--allow-unknown-network): Add internal variable to
allow IRC session to continue despite the network being unknown.
(erc-networks--set-name): Tell the user about falling back to a given
ID when the network can't be determined.  When that's so, end the
session by destroying the connection unless
`erc-networks--allow-unknown-network' is enabled.  (Bug#59976.)
(erc-networks--ensure-announced): Include the fallback announced
server name in the error message.
* test/lisp/erc/erc-networks-tests.el (erc-networks--set-name): Add
dummy server process and don't expect an error to be signaled.
* test/lisp/erc/erc-scenarios-misc.el
(erc-scenarios-networks-announced-missing): Don't expect an error to
be signaled.
* test/lisp/erc/resources/networks/announced-missing/foonet.eld:
Remove "mode" match pattern.