Intermittent prompt_test failure

e.g., https://github.com/Shopify/cli-ui/actions/runs/5314630194/jobs/9622054658?pr=429

Run options: --seed 63275

# Running:

........................................................F............................S...
rake aborted!

Command failed with status (1)
Finished in 23.282313s, 3.8226 runs/s, 11.[7](https://github.com/Shopify/cli-ui/actions/runs/5314630194/jobs/9622054658?pr=429#step:4:8)6[8](https://github.com/Shopify/cli-ui/actions/runs/5314630194/jobs/9622054658?pr=429#step:4:9)6 assertions/s.
/Users/runner/work/cli-ui/cli-ui/vendor/bundle/ruby/3.0.0/gems/rake-13.0.6/exe/rake:27:in `<top (required)>'

/Users/runner/hostedtoolcache/Ruby/3.0.6/x64/bin/bundle:23:in `load'
  1) Failure:
/Users/runner/hostedtoolcache/Ruby/3.0.6/x64/bin/bundle:23:in `<main>'
CLI::UI::PromptTest#test_confirm_sigint [/Users/runner/work/cli-ui/cli-ui/test/cli/ui/prompt_test.rb:26]:
Tasks: TOP => default => test
(See full trace by running task with --trace)
Expected "" to include "sentinel".

8[9](https://github.com/Shopify/cli-ui/actions/runs/5314630194/jobs/9622054658?pr=429#step:4:10) runs, 274 assertions, 1 failures, 0 errors, 1 skips

I tried TESTOPTS="--seed=63275" bundle exec rake on my Macbook, but it's passing there.

Start with test/cli/ui/prompt_test.rb at line 26 and reproduce the reported seed with TESTOPTS="--seed=63275" bundle exec rake. Compare the CI failure, where CLI::UI::PromptTest#test_confirm_sigint expected "sentinel", with the passing MacBook run. Done means the prompt test no longer fails intermittently in the reported test run.