Testing Directory traversal/file include (OTG-AUTHZ-001)

This is a draft of a section of the new Testing Guide v3

Brief Summary
..here: we describe in "natural language" what we want to test.

Description of the Issue
...here: Short Description of the Issue: Topic and Explanation

Black Box testing and example
Testing for Topic X vulnerabilities: ... Result Expected: ...

Gray Box testing and example
Testing for Topic X vulnerabilities: ... Result Expected: ...