Update tests.md (Azure#22848)

* Update tests.md

* Update doc/dev/tests.md

Co-authored-by: Johan Stenberg (MSFT) <[email protected]>

* Mention proxy and guide; tweaks

* Update test setup instructions (in progress)

* Updated test resource info (in progress)

* New test structure (in progress)

* Add references to test-resources.bicep 

Infrastructure supports ARM templates (.json) and BICEP files (.bicep).

* Almost final updates (in progress)

Co-authored-by: Johan Stenberg (MSFT) <[email protected]>
Co-authored-by: McCoy Patiño <[email protected]>

Author: 3 people