Fix the gets text content, with correct properties (issue 8276) unit test

This unit test failed consistently in Firefox both locally and on GitHub
Actions (but not in Chrome or on the bots), which suggests a timing issue.

Since all other unit tests that rely on `commonObjs` actually render the
page, most likely to make sure that `commonObjs` is fully populated at
the time of the check, this commit mirrors that approach to this test,
which indeed fixes the issue.

test/unit/api_spec.js

@@ -3723,9 +3723,25 @@ page 1 / 3`);
         vertical: false,
       });
 
-      // Wait for font data to be loaded so we can check that the font names
-      // match.
-      await pdfPage.getOperatorList();
+      // Render the page to make sure that the font data is loaded so we can
+      // check that the font name in `commonObjs` matches.
+      const { canvasFactory } = pdfDoc;
+      const viewport = pdfPage.getViewport({ scale: 1 });
+
+      const canvasAndCtx = canvasFactory.create(
+        viewport.width,
+        viewport.height
+      );
+      const renderTask = pdfPage.render({
+        canvas: canvasAndCtx.canvas,
+        viewport,
+      });
+
+      await renderTask.promise;
+      // The canvas is no longer necessary, since we only care about the
+      // `commonObjs` property being populated below.
+      canvasFactory.destroy(canvasAndCtx);
+
       expect(pdfPage.commonObjs.has(fontName)).toEqual(true);
 
       await loadingTask.destroy();