Added unit tests for the w.gl.utils.backOff promise

What does this MR do?

Improves the code coverage for the w.gl.utils.backOff promise

Are there points in the code the reviewer needs to double check?

Maybe the timeout values for the async tests

Why was this MR needed?

To improve code coverage on the karma/jasmine unit tests suite

Screenshots (if relevant)

Before

After

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together