Fix formatting code block in TESTING.rst

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[ad-m]

@potiuk , could you review that and provide any comments? I have been informed that you have the widest knowledge of testing in the entire Apache Airflow project.

[potiuk]

Stylelint complains - it's good but the style problems need to be fixed.

[mik-laj]

It does not look good.

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It does not look good.

You looking on section which I don't modify. It was out of scope of that PR. I decided to extend scope of that PR to include that section.

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@potiuk , @mik-laj , thanks for you help!