Prefer 'font-lock-face to 'face in erc where appropriate

Prefer 'font-lock-face to 'face in erc where appropriate

* lisp/erc/erc-button.el (erc-button-add-face): Prefer
'font-lock-face to 'face where appropriate.
* lisp/erc/erc-capab.el (erc-capab-identify-add-prefix)
* lisp/erc/erc-dcc.el (erc-dcc-chat-parse-output)
* lisp/erc/erc-goodies.el (erc-controls-propertize)
* lisp/erc/erc-stamp.el (erc-format-timestamp)
* lisp/erc/erc-track.el (erc-faces-in)
* lisp/erc/erc.el (erc-load-irc-script-lines, erc-display-msg)
(erc-display-command, erc-make-notice, erc-highlight-notice)
(erc-format-my-nick, erc-format-@nick, erc-format-privmessage)
(erc-display-prompt, erc-display-message-highlight)
(erc-log-irc-protocol): Ditto.

* test/lisp/erc/erc-track-tests.el: Converted asserts into ert
tests.