Fix "behavior" typo in TESTING.md

[romainkomorn-exdatadog]

What does this PR do?

Fixes #1711 about a typo in TESTING.md where "behaviro" should be "behavior"

What inspired you to submit this pull request?

I looked at the TESTING.md and saw it.

Additional Notes

N/A

Review checklist

• This PR includes all newly recorded cassettes for any modified tests.
• This PR does not rely on API client schema changes.
  • The CI should be fully passing.

[romainkomorn-exdatadog]

I'm skipping the changelog because I think I'm already being silly enough with #1711 as is..

[romainkomorn-exdatadog]

/merge

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