Fix sticker tests

* Makefile (check-%): Accept extra SELECTOR arg
(check-fancy): `debug-on-error' is useless.

* contrib/sly-mrepl.el (sly-mrepl--teardown): Cleanup `sly-mrepl--channel'
property in dedicated stream process.
(sly-mrepl--dedicated-stream-output-filter): Protect against
killed buffers.

* test/sly-stickers-tests.el (sly-stickers--call-with-fixture):
Reorganize macro helper.
(sly-stickers--face-p): Make logic explicit.
(stickers-record-stuff): Remove useless "coiso" sticker.
(stickers-record-stuff): Add extra check and sync.