Testing: workaround TypeScript compiler limitation

It appears that

    assert.equal(lastArgs?.type, 'dirty', 'setDirty/isDirty');

causes error

    error TS2339: Property 'type' does not exist on type 'never'.

I think this is actually a bug in tsc:

    microsoft/TypeScript#11498

So this should be considered a workaround only. Alternative solution
would be to annotate this line as @ts-ignore.

modules/tinymce/src/core/test/ts/browser/EditorTest.ts

@@ -401,7 +401,7 @@ describe('browser.tinymce.core.EditorTest', () => {
     assert.isFalse(editor.isDirty(), 'setDirty/isDirty');
 
     editor.setDirty(true);
-    assert.equal(lastArgs?.type, 'dirty', 'setDirty/isDirty');
+    assert.equal((lastArgs as unknown as EditorEvent<{}>).type, 'dirty', 'setDirty/isDirty');
     assert.isTrue( editor.isDirty(), 'setDirty/isDirty');
 
     lastArgs = undefined;