Add wasm test that input prompt using line not character buffering for stdin

[ianthomas23]

The correct handling in WebAssembly of the input prompt used for commit messages (issue #44) was fixed in cockle in jupyterlite/cockle#309. This PR adds a test to confirm the fix by using a backspace character in the input prompt to delete the previous character, thus confirming that stdin is using line buffering not individual character buffering.

The new test is disabled if not running in WebAssembly. It already worked correctly in a manual test but running it via python's subprocess.run doesn't work without us using something more complicated like an input pipe to send the stdin characters, and that approach cannot work in WebAssembly so we'd have to implement some new workaround and it is not worth the effort.

Closes #44.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.27%. Comparing base (5ec6e88) to head (8816c78).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #146      +/-   ##
==========================================
+ Coverage   88.22%   88.27%   +0.05%     
==========================================
  Files          61       61              
  Lines        3065     2730     -335     
  Branches      330      330              
==========================================
- Hits         2704     2410     -294     
+ Misses        361      320      -41     

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