Update testing_packages.rst

[shahar1]

After trying to run breeze start-airflow --mount-sources remove --use-packages-from-dist without much success, I figured out from the code that it's necessary also to supply --use-airflow-version wheel/sdist. I updated the docs to reflect this and made some other improvements.

cc: @potiuk

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Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
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In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in newsfragments.

[potiuk]

Thanks!