Fix some sly-stickers bugs in code and tests

* contrib/sly-stickers.el (sly-stickers--last-value-desc): New function.
(sly-stickers--populate-sticker)
(sly-stickers--mark-empty-sticker): Use it.

* test/sly-stickers-tests.el (sly-stickers--call-with-fixture):
Use :CL explicitly.
(stickers-basic-navigation, stickers-should-stick)
(stickers-when-invalid-dont-stick): Don't need the defpackage
forms here. Fix doncstrings.