Fix console error.

I think the handler wasn’t getting removed after the check, so it was failing on the next signal emission. testEmission automatically removes the handler after the check, taking care of this issue.

tests/test-docregistry/src/mimedocument.spec.ts

@@ -17,7 +17,11 @@ import {
 
 import { RenderedText, IRenderMime } from '@jupyterlab/rendermime';
 
-import { createFileContext, defaultRenderMime } from '@jupyterlab/testutils';
+import {
+  createFileContext,
+  defaultRenderMime,
+  testEmission
+} from '@jupyterlab/testutils';
 
 const RENDERMIME = defaultRenderMime();
 
@@ -111,10 +115,10 @@ describe('docregistry/mimedocument', () => {
       it('should change the document contents', async () => {
         RENDERMIME.addFactory(fooFactory);
         await dContext.initialize(true);
-        let called = false;
-        dContext.model.contentChanged.connect(() => {
-          expect(dContext.model.toString()).to.equal('bar');
-          called = true;
+        const emission = testEmission(dContext.model.contentChanged, {
+          test: () => {
+            expect(dContext.model.toString()).to.equal('bar');
+          }
         });
         const renderer = RENDERMIME.createRenderer('text/foo');
         const widget = new LogRenderer({
@@ -125,7 +129,7 @@ describe('docregistry/mimedocument', () => {
           dataType: 'string'
         });
         await widget.ready;
-        expect(called).to.equal(true);
+        await emission;
       });
     });
   });