Write Gherkin scenarios for VeraCrypt automated tests

This is the initial part of “A.3 Write automated tests”. Actual steps will be implemented in #14470 (closed), #14471 (closed) and #14472 (closed).

For potential scenario candidates, see:

• our end-user doc
• the scope of our work
• our UI design

Feature Branch: test/14471-veracrypt

Parent Task: #14468

Related issues

• Blocked by #14742 (closed)
• Blocks #14471 (closed)

Original created by @segfault on 14469 (Redmine)