Passing a single Node to mermaid.init() results in an error, as it
calls `new NodeList()`, which causes `TypeError: Illegal constructor`.
See 5597cf45bf/src/mermaid.ts (L73)
If we instead use the `ArrayLike` interface, we can just use a simple
array, instead of a NodeList.
I've also added a basic test case, by mocking the `mermaidAPI.render()`
function so it isn't called, as the d3 functions don't work in Node.JS.
The mocks are a bit messy, since
a) Jest doesn't fully support ESM yet, and
b) mermaidAPI is frozen with `Object.freeze()`,
but the mermaidAPI mocks work as long as we keep them simple.
Fixes: c68ec54fdd