test: extract message-capture helper into testutil

Five test files hand-rolled the same scaffolding to capture the echo-area message: a nil-initialized var, a cl-letf on message, and a lambda that stored the formatted string. Centralize it as testutil-wttrin-with-captured-message, which binds the var and captures the last message shown.

Behavior is unchanged. The full suite stays green. Sites that mock message for a different reason are left alone: the ones that silence it with #'ignore, and the one that accumulates every message into a list.

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