feat: adds --test flag in yr fmt

[shellcromancer]

Adds -t/--test flag in yr fmt which returns an error code if formatting made changes on the file.

This feature exists in a few other language formatting tools (jsonnet, terraform, gofmt) which is useful as part of stylistic CI/CD checks

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[shellcromancer]

Signed the Google CLA, should be ready for a maintainer review, thanks! 👍

[plusvic]

In https://github.com/VirusTotal/yara-x/commit/4b0889938df1feff849e6c9cdd4d84ab59eeec86 I made multiple changes to the fmt command. Now it always exits with error code 0 if files didn't change or 1 if otherwise.

[shellcromancer]

Makes sense, thanks for including the feature in your changes. 👍