tests: add regression coverage for run_vulkan launcher selection

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Summary

Add deterministic regression coverage for the Vulkan launcher script path/arch selection logic.

Risky behavior now covered

• scripts/run_vulkan.sh launched from scripts/ should prefer sibling release/idtech3 (not a same-dir fallback copy).
• On arm64/aarch64, launcher should execute release/idtech3.aarch64 when present.
• On arm64/aarch64, launcher should reliably fall back to release/idtech3 when idtech3.aarch64 is absent.
• Engine command-line arguments are forwarded unchanged.

Test files added/updated

• Added: tests/scripts/test_run_vulkan.sh
• Updated: CMakeLists.txt (register test_run_vulkan_launcher in CTest with labels unit;regression;scripts)

Why this materially reduces regression risk

run_vulkan.sh is a shared entrypoint used by ARM and desktop Vulkan launch workflows. Recent production changes modified binary discovery semantics; regressions here can silently launch the wrong binary or fail on ARM devices. These tests exercise the highest-risk branching logic in an isolated temp layout with stubbed binaries/uname, making them deterministic and fast while directly validating real launch decisions.

Validation

• tests/scripts/test_run_vulkan.sh scripts/run_vulkan.sh
• ctest -R test_run_vulkan_launcher --output-on-failure (from build-vk-Release after reconfigure)