Add test for erc-cmd-SQUERY

Add test for erc-cmd-SQUERY

* lisp/erc/erc-backend.el (erc-message): Revise doc string.
* test/lisp/erc/erc-scenarios-base-misc-regressions.el (erc-cmd-MOTD):
Move test to another file specifically for slash commands.
* test/lisp/erc/erc-scenarios-misc-commands.el: New file.
* test/lisp/erc/resources/base/commands/motd.eld: Move file elsewhere.
* test/lisp/erc/resources/commands/motd.eld: "New" file, moved here
reusing the same Git blob from now deleted subdir base/commands.
* test/lisp/erc/resources/commands/squery.eld: New file.  (Bug#67209)