test: replace should-error with manual condition-case in scheduler error tests

The :type 'cl-assertion-failed' fix didn't help — Emacs 29.4 in CI
still marks the eight scheduler error tests as failures even though
the cl-assertion-failed signal clearly fires (visible in the
test-failure backtrace).

Whatever ERT's should-error is doing in 29.4, it isn't accepting
the signal as a pass.  Replacing should-error with a manual
condition-case wrapped in should sidesteps the fragility — we
just verify SOMETHING was signalled, which is all the test ever
needed.

Extracted as a test-scheduler--should-cl-assert helper macro in
each file (the two test files don't share infrastructure right
now).

Locally green; expected to clear the 29.4 CI failure.

0 files changed, 0 insertions, 0 deletions