Fix a previously silent test failure

Fix a test that was failing but failing in a way that went unreported
previously due to a combination of it occurring after the test function returned
because of a missing `await` call and because of the issue with uncaught
exceptions between tests described in https://github.com/hypothesis/client/issues/2249.

This test was failing but the failure was only reported after the test
function returned due to a missing `await`.

src/sidebar/services/test/load-annotations-test.js

@@ -282,9 +282,15 @@ describe('loadAnnotationsService', () => {
         references: ['parent_annotation_1', 'parent_annotation_2'],
       },
     ];
-    it('clears annotations from the store first', () => {
+    it('clears annotations from the store first', async () => {
+      fakeApi.annotation.get.onFirstCall().resolves({
+        id: 'target_annotation',
+        references: [],
+      });
       const svc = createService();
-      svc.loadThread('target_annotation');
+
+      await svc.loadThread('target_annotation');
+
       assert.calledOnce(fakeStore.clearAnnotations);
     });