Bug 1039825 - "ReferenceError: newTab is not defined" running `test-widget`

- Added `.catch(assert.fail)` after each `openNewWindowTab` calls
- Removed unused and undeclared `newTab` variable
Merge pull request #1553 from ZER0/test-widget/1039825

fix Bug 1039825 - "ReferenceError: newTab is not defined" running `test-widget` r=@erikvold