tests: deduplicate environment check output helper

[loulanyue]

Relates-to: #3155

Summary

• move the duplicated environment check_output mock into tests.helpers
• preserve the console tests' /usr base prefix and the environment manager tests' runtime sys.base_prefix
• import the shared helper directly from each affected test module

This removes 34 net lines while keeping the existing test behavior unchanged.

Verification

• pytest -n 0 tests/console/commands/env/test_use.py tests/console/commands/env/test_remove.py tests/utils/env/test_env_manager.py (68 passed)
• mypy tests/helpers.py tests/console/commands/env/helpers.py tests/console/commands/env/test_remove.py tests/console/commands/env/test_use.py tests/utils/env/test_env_manager.py
• pre-commit run --files tests/helpers.py tests/console/commands/env/helpers.py tests/console/commands/env/test_remove.py tests/console/commands/env/test_use.py tests/utils/env/test_env_manager.py

The local Homebrew environment provides python3 but no bare python, so the pytest run used a temporary PATH entry mapping python to python3. No repository files were changed for that local environment difference.

Checklist

• Updated and ran the affected tests.
• Updated documentation (not applicable; test-only cleanup).

[sourcery-ai]

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