Add test for erc-extract-command-from-line

Add test for erc-extract-command-from-line

* lisp/erc/erc.el (erc-extract-command-from-line): Redo doc string.
* test/lisp/erc/erc-tests.el (erc--parse-isupport-value): Add case for
commonly seen escaped character ?=.
(erc-extract-command-from-line): New test.
; * test/lisp/erc/resources/erc-d/resources/basic.eld: Update.
; Don't send unnegotiated multi-prefixed userhost names.
; * test/lisp/erc/resources/erc-d/resources/dynamic-barnet.eld: Update.
; * test/lisp/erc/resources/erc-d/resources/dynamic-foonet.eld: Update.
; * test/lisp/erc/resources/erc-d/resources/dynamic.eld: Update.
; * test/lisp/erc/resources/erc-d/resources/eof.eld: Update.
; * test/lisp/erc/resources/erc-d/resources/fuzzy.eld: Update.
; * test/lisp/erc/resources/erc-d/resources/incremental.eld: Update.
; * test/lisp/erc/resources/erc-d/resources/linger.eld: Update.
; * test/lisp/erc/resources/erc-d/resources/no-block.eld: Update.
; * test/lisp/erc/resources/erc-d/resources/no-match.eld: Update.
; * test/lisp/erc/resources/erc-d/resources/unexpected.eld: Update.

(cherry picked from commit 13c7933a9d4b26e74e7f5e19d70bb89003239c34)