Fix missing asserts for GDK::Output.puts

What does this merge request do and why?

This minor MR fixes formatting anomalies with make rspec due to a missing assertion on GDK::Output.puts and aims to halt "broken windows" 🙂

main

$ make rspec
Bundle check passed.
RSpec: ..............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................
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Finished in 3.5 seconds (files took 0.52716 seconds to load)
1032 examples, 0 failures

ashmckenzie/minor-spec-fixes (this branch)

$ make rspec
Bundle check passed.
RSpec: ........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

Finished in 4.48 seconds (files took 0.54071 seconds to load)
1032 examples, 0 failures

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 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.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.