BUG: Fix mermaid.ink visual comparison test sensitivity to environment

[Carwyn Pelley (cpelley)]

Opted for simply checking that images are successfully returned by mermaid.ink and that such files are non-zero in size.

Can't justify adding and maintaining code to DAGrunner testing for handling tolerant image comparison (svg/png/jpg).

Image generation is not operational or even part of the visualisation UI so doing anything more than checking that mermaid.ink actually returns a file cannot be justified.

Background

Mermaid cannot return consistent results since it works by dynamically generating a users diagram -- floating-point value precision differences leading to logic differences with layout.
Even svg element IDs are themselves allocated. We can enforce a consistent seed but this has its own implications on svg support.

Issues

• Closes TEST: Failing image comparison in some environments and not others #98

Note

Markdown file (.md) generation (which we do care about) continues to be fully tested (i.e. content verified against reference file).