Turnt

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- A new `out_dir` directory tells Turnt where to put its expected output files.

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- The diff output, when `--diff` is enabled, now goes to stderr instead of stdout. This makes it possible to redirect diffs separately from the test results.

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- `-h` is now an alias for `--help`.
- `differing` and `missing` notes are now printed on the same line as the test itself, as TAP directives. This should both be easier to read when looking at the TAP output directly, and it should make life a bit simpler for TAP consumers.
- A new `binary` config option that disables looking with test files for (text) overrides. Even without this flag, binary files no longer crash Turnt altogether and instead merely log an error message.

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- Add support for multiple *test environments* that run different commands on the same file. This is especially useful for differential testing, when multiple commands have the same expected output.
- Flush the output buffer after every line, which makes streaming TAP consumers more useful.

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- Search for `turnt.toml` configuration files in ancestor directories, not just in the same directory as the test.
- The minimum Python version advanced to 3.6.
- Switch the TOML library to [tomli][] (or the standard library on Python 3.11+).

[tomli]: https://github.com/hukkin/tomli

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