Fix tests sly-parse for upcoming 24.4

In Emacs 24.4 @ doesn't have "'" syntax anymore. We could add it, but
perhaps it's better to fix `sly-parse''s regexp, which should be going
away in favour of `syntax-ppss' anyway.

* contrib/sly-parse.el (sly-parse-form-until): Unbreak a test for Emacs 24.4

* contrib/test/sly-parse-tests.el (form-up-to-point.1): Don't use
`skip-test-p'