Fix pristine spec is stuck in TTY

What does this merge request do and why?

Fix pristine spec is stuck in TTY

This fixes a bug in the gdk pristine spec where if we're running RSpec from a TTY, the $stdin.tty? check being true check will cause it to wait for user input.

We just mock the $stdin.tty? check to always return false.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This MR references an issue describing the change.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.