Turnt

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- Add comments to the TAP output to indicate detected output differences.
- Add a `--config` command-line option.

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- Run tests in parallel with `-j`.

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- Support options in directory tests (the `opts_file` config option).
- Outputs from directory tests go to `dirname/out.ext` instead of
`dirname/dirname.ext`.

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- The diff command is now configurable.
- We use a unified diff by default.

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- A new `--args` option lets you override the `args` field that otherwise comes from in-file settings.
- You can now capture the standard error from a command. In the same way that the `-` pseudo-file indicates the standard output, `2` now indicates the standard error.
- Tests can now have (non-zero) expected return codes. Use `return_code` in the configuration file or `RETURN` in file comments.

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A new `--print` option enables a convenient mode for debugging tests that just shows the output instead of checking any results.