Create test for mTLS Fetcher

[JPeer264]

Description

mTLS' bindings are as of the docs only a Fetcher: https://developers.cloudflare.com/workers/runtime-apis/bindings/mtls/

We need to add a test to verify its support

[linear-code]

JS-2465

[github-actions]

A PR closing this issue has just been released 🚀

This issue was referenced by PR #21569, which was included in the 10.59.0 release.