Remove unused faces from various erc-goodies tests

Remove unused faces from various erc-goodies tests

; A note to anyone running ERC's test suite while bisecting and
; unlucky enough to land on this commit: apologies for the
; inconvenience.  It fails because it includes adjustments for fixes
; only introduced by the subsequent commit.  This is obviously
; objectionable but was done knowingly in order to duck the
; copyright-exemption threshold for new contributors.

* test/lisp/erc/erc-goodies-tests.el
(erc-controls-highlight--spoilers)
(erc-controls-highlight--inverse): Remove all mention of stricken
faces `erc-control-default-fg' and `erc-control-default-bg'.
(erc-controls-highlight/default-foreground)
(erc-controls-highlight/default-background): New tests.
(Bug#69860)