test(gpu): Register GPU tests with CTest

Changed GPU test targets from add_demo_executable to add_demo_test:
- test_effect_base → EffectBaseTest (Test #24)
- test_demo_effects → DemoEffectsTest (Test #25)
- test_post_process_helper → PostProcessHelperTest (Test #26)

Now all GPU tests run automatically with 'ctest' command.
Total test count: 23 → 26 tests (all passing)

Phase 2 GPU testing infrastructure complete and integrated into CI.

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