Remove commented-out test code that is no longer needed

[Copilot]

Addresses feedback from PR #436 review to remove commented-out code in test_mypy_target_dry_run.

Changes

• Removed commented-out directory setup and environment configuration code (8 lines)
• Code became redundant when src/src.mk was added to SPLIT_MAKEFILES - the fixture now creates the src/ directory automatically when copying split makefiles

Context

The test fixture setup_tmp_makefile creates parent directories for all split makefiles, making the manual src/ directory creation and SOURCE_FOLDER environment variable setup unnecessary.

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[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

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• astral.sh
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If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
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