Fix a wrong `X-Gitlab-Event` header when testing webhooks

Merged Rémy Coutable requested to merge 37288-fix-wrong-header-when-testing-webhook into master

What does this MR do?

This fixes a wrong X-Gitlab-Event header when testing a webhook:

Are there points in the code the reviewer needs to double check?

Not that I can think of.

Why was this MR needed?

Because the "Testing webhook" feature broke inadvertently.

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes #37288 (closed)

Edited by Rémy Coutable

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