test: shadow signal-hook-function so cl-assertion-failed can be caught

Attempt 4 at making the eight scheduler error tests pass on
Emacs 29.4 in CI.

The earlier approaches kept failing because ERT 29.4 installs
ert--should-signal-hook as signal-hook-function around the entire
ert-deftest body — not just inside should forms.  That hook fires
on every signal before any inner condition-case can catch it,
which is why even a bare (condition-case ... (cl-assertion-failed
nil)) at the top of the test body didn't work.

The new helper rebinds signal-hook-function to nil inside its own
let-scope, so condition-case catches the cl-assertion-failed
signal normally.  The ert-fail on the no-error path runs outside
that shadowing scope, so it still routes through ERT's failure
handling.

Locally green; pushing to test 29.4 in CI.

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