test gets in task

[emiltin]

Adds a test for #256

Test can be run with:

% bundle exec sus test/async/task.rb:812                             
1 passed out of 1 total (1 assertions)
🏁 Finished in 723.0µs; 1383.126 assertions per second.
🐇 No slow tests found! Well done!

It succeeds on Mac, but I suspect it will hang on Windows, as it's essentially the example from #256.

Types of Changes

• added a test

Contribution

• I added tests for my changes.
• I tested my changes locally. (not on Windows though..)
• I agree to the Developer's Certificate of Origin 1.1.

[ioquatix]

We should probably try to add Windows to the CI matrix.