Use advertised PREFIX when formatting nicks in ERC

Use advertised PREFIX when formatting nicks in ERC

* lisp/erc/erc-speedbar.el (erc-speedbar-insert-user): Run
`erc-get-channel-membership-prefix' in associated buffer if possible.
* lisp/erc/erc.el (erc-get-channel-membership-prefix): Use known
prefix mappings when determining status chars.
* test/lisp/erc/erc-tests.el (erc--parsed-prefix): Use common helpers
for initializing buffers, and use a more realistic example for PREFIX
value.
(erc--update-channel-modes): Add current buffer to `erc-server-user'
object to maintain essential invariant, even though this doesn't
affect the test's outcome.
(erc-tests--equal-including-properties): Move to `erc-tests-common'
and rename `erc-tests-common-equal-with-props'.
(erc--merge-prop, erc--remove-from-prop-value-list,
erc--remove-from-prop-value-list/many): Use new name for
`erc-tests-common-equal-with-props'.
(erc-get-channel-membership-prefix): New test.
(erc--determine-speaker-message-format-args,
erc--determine-speaker-message-format-args/queries-as-channel,
erc--determine-speaker-message-format-args/queries): Use new name
for `erc-tests-common-equal-with-props'.
* test/lisp/erc/resources/erc-tests-common.el
(erc-tests-common-equal-with-props): New macro, originally
`erc-tests--equal-including-properties' from erc-tests.el.
(erc-tests-common-make-server-buf): Initialize tables and make NAME
argument optional.  (Bug#67677)