tidy: add unit tests for alphabetical markers.

This requires introducing `tidy_error_ext!` as an alternative to
`tidy_error!`. It is passed a closure that is called for an error. This
lets tests capture tidy error messages in a buffer instead of printing
them to stderr.

It also requires pulling part of `check` out into a new function
`check_lines`, so that tests can pass in an iterator over some strings
instead of a file.

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