Revert "[Sheriff] Disable flaky ChromeVoxBackgroundTest.SelectSingleBasic"

This reverts commit ae012e64.

Reason for revert: The culprit CL was reverted.

Original change's description:
> [Sheriff] Disable flaky ChromeVoxBackgroundTest.SelectSingleBasic
> 
> TBR=akihiroota
> 
> Bug: 1042673
> Change-Id: Iccf2064ee5b817bc9e7f12c026cd8569e39d22b7
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2002816
> Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
> Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#732396}

TBR=jdoerrie@chromium.org,akihiroota@chromium.org

Change-Id: I1c098ec50bd7feb5d3d0c0780bc01a094b087570
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1042673
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2006831Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732693}

chrome/browser/resources/chromeos/accessibility/chromevox/background/background_test.js

@@ -262,8 +262,7 @@ TEST_F(
       });
     });
 
-// Flaky: https://crbug.com/1042673
-TEST_F('ChromeVoxBackgroundTest', 'DISABLED_SelectSingleBasic', function() {
+TEST_F('ChromeVoxBackgroundTest', 'SelectSingleBasic', function() {
   var mockFeedback = this.createMockFeedback();
   this.runWithLoadedTree(this.formsDoc, function() {
     mockFeedback.expectSpeech('apple', 'has pop up', 'Collapsed')