Add probing erc-server-reconnect-function variant

Add probing erc-server-reconnect-function variant

* lisp/erc/erc-backend.el (erc-server-reconnect-timeout): Replace
questionable claim with recommendation for alternate value when
experiencing nick rejections.
(erc-server-reconnect-function): Add new, somewhat experimental value
`erc-server-delayed-check-reconnect'.
(erc--server-connect-function): Add variable for process-dialing
monitor, a function.
(erc--server-propagate-failed-connection): Add function to serve as
default monitor to run on process creation and maybe execute failure
handlers.
(erc-server-connect): Run `erc--server-connect-function' for async
processes one second after creation.
(erc--server-reconnect-timeout, erc--server-reconnect-timeout-check,
erc--server-reconnect-timeout-scale-function,
erc--server-reconnect-timeout-double): Add supporting variables and
functions for `erc-server-delayed-check-reconnect'.
(erc-server-delayed-check-reconnect): Add possible alternate value for
option `erc-server-reconnect-function' that only attempts to reconnect
after hearing back from the server.
(erc-schedule-reconnect): Ensure previous `erc-server-process' is
deleted.
* test/lisp/erc/erc-scenarios-base-auto-recon.el: New file.
* test/lisp/erc/resources/base/reconnect/just-eof.eld: New file.
* test/lisp/erc/resources/base/reconnect/just-ping.eld: New file.
* test/lisp/erc/resources/base/reconnect/ping-pong.eld: New file.
* test/lisp/erc/resources/base/reconnect/unexpected-disconnect.eld:
New file.
* test/lisp/erc/resources/erc-scenarios-common.el
(erc-scenarios-common--make-bindings): Shadow
`timer-list'.  (Bug#62044.)