Use Mermaid for hierarchy in `Tests/Support`

[arogachev]

Reasons:

• XML is harder to maintain.
• Image needs to be regenerated on every update.

Files:

• https://github.com/yiisoft/rbac/blob/a4f96ac2bb77003d9c04cfb424b05a5138ec5041/tests/Support/hierarchy.graphml
• https://github.com/yiisoft/rbac/blob/a4f96ac2bb77003d9c04cfb424b05a5138ec5041/tests/Support/hierarchy.png