MaterialX Graph Editor Needs Unit Tests

[kwokcb]

Issue

The graph editor should have basic unit testing to ensure a minimal trust level for users.
Currently testing is purely manual at time of code commit which can result in unintended data loss, and/or regressions in
behaviour or visual presentation.

Proposal

Even if it's a interactive tool these areas should be testable, though the amount of work varies.

1. Data integrity : Load / save of documents (tests conversion to/from MTLX to UI graph data model)
2. Operation integrity : Operations always behave correctly.
3. Visual display integrity : The UI look does not regress.
4. Interaction integrity : All interactions work as expected.

Staged Implementation

1. Should be simple if "save" is a command option.
2. This requires 1, and a way to invoke operations.
3. Can be done with snapshot on graph, property editor (not hard to do) as a command option.
4. The operations performed are covered by 2. but this would be a bit harder to do as event triggers are required.

Thus propose to tackle at least 1 and 3 as simple additions.