test: fix initThrowsErrors test with esbuild

Jest mocks don't work well with ESM (especially since esbuild
is stricter with ESM code than jest is).

We can convert this test to instead import the mermaid modules
using `require('./mermaid')`, so that jest can easily mock it.

src/mermaid.spec.js

@@ -1,15 +1,16 @@
-import mermaid from './mermaid';
-import { mermaidAPI } from './mermaidAPI';
-import flowDb from './diagrams/flowchart/flowDb';
+// mocks the mermaidAPI.render function (see `./__mocks__/mermaidAPI`)
+jest.mock('./mermaidAPI');
+// jest.mock only works well with CJS, see https://github.com/facebook/jest/issues/9430
+const { default: mermaid } = require('./mermaid');
+const { mermaidAPI } = require('./mermaidAPI');
+const { default: flowDb } = require('./diagrams/flowchart/flowDb');
+
 import flowParser from './diagrams/flowchart/parser/flow';
 import flowRenderer from './diagrams/flowchart/flowRenderer';
 import Diagram from './Diagram';
 
 const spyOn = jest.spyOn;
 
-// mocks the mermaidAPI.render function (see `./__mocks__/mermaidAPI`)
-jest.mock('./mermaidAPI');
-
 afterEach(() => {
   jest.restoreAllMocks();
 });
@@ -54,6 +55,8 @@ describe('when using mermaid and ', function () {
       node.appendChild(document.createTextNode('graph TD;\na;'));
 
       mermaid.initThrowsErrors(undefined, node);
+      // mermaidAPI.render function has been mocked, since it doesn't yet work
+      // in Node.JS (only works in browser)
       expect(mermaidAPI.render).toHaveBeenCalled();
     });
   });