test/spec_helper: prevent stdin trashing

[Bo98]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

This fixes a situation where the output of brew tests will modify the stty attributes of the terminal and also cause test output to appear garbled with mishandled newlines.

Not entirely sure why this happens - I can't reproduce it outside of brew tests - but we should be doing this safe handling anyway, and we already to it for stdout and stderr.

[BrewTestBot]

Review period skipped due to critical label.

[MikeMcQuaid]

Makes sense, thanks @Bo98!