Fix selected test subprocess buffering

[puneetdixit200]

Summary

• Force Cricket's async test executor subprocess pipes to use unbuffered binary mode.
• Add a regression test that verifies selected test execution passes bufsize=0 when creating the backend subprocess.
• Repair the GitHub Actions workflow by moving off unavailable Python/action versions, using Toga's dummy backend for tests, pinning compatible Django/pytest/Toga test dependencies, and running unittest discovery directly.

Closes #91.

Verification

• env/bin/python -m unittest tests.test_executor.ExecutorTests.test_subprocess_uses_unbuffered_binary_pipes
• env/bin/python -m unittest tests.test_executor tests.test_unittest tests.test_models
• PATH=$PWD/env/bin:$PATH TOGA_BACKEND=toga_dummy env/bin/python -m unittest tests.test_pytest
• Selected-folder smoke test using UnittestTestSuite.find_tests(labels={'tests.submodule'}) and Executor.run()
• env/bin/python -m py_compile setup.py cricket/executor.py tests/test_executor.py
• git diff --check

AI-assisted work

This change was prepared with AI assistance and manually reviewed.

[freakboy3742]

Closing as part of a flood of AI generated content from the same user.