Store one string per user in erc--spkr msg prop

Store one string per user in erc--spkr msg prop

* lisp/erc/erc.el (erc--msg-props): Mention that the `erc--spkr'
msg-prop value is taken from the `nickname' slot of the user's
`erc-server-users' entry.
(erc--speakerize-nick): Avoid using the provided NICK parameter for
the `erc--spkr' property.  Instead, use the version from the
`nickname' slot of its `erc-server-users' item, which is itself an
`erc-server-user' object.  These text props were originally introduced
in ERC 5.6 as part of bug#67677.
* test/lisp/erc/erc-tests.el (erc--refresh-prompt)
(erc--check-prompt-input-functions, erc-send-current-line)
(erc--check-prompt-input-for-multiline-blanks)
(erc-send-whitespace-lines): Use more convenient helper utility to
create fake server buffer where possible.
(erc--speakerize-nick): New test.
* test/lisp/erc/resources/erc-tests-common.el
(erc-tests-common-make-server-buf): Don't use ERT temp buffer's name
for dialed server, etc., because it contains unwanted chars.
(erc-tests-common-with-process-input-spy): Defer to each test to set
up its own prompt, etc.  (Bug#72736)

(cherry picked from commit b0ebb82076315f8e50159aff6caded4c5ee4438c)