Implement stub_do_not_track helper

What does this MR do?

This helper can be used whenever there's a need to stub the DNT request header.

The TrackingHelpers module needs to be included in a spec for it to work.

No changelog entry needed since this is only being used for specs.

Does this MR meet the acceptance criteria?

Conformity

• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)

Closes #295264 (closed)