CMake: Fix old mistake to make ctest pass independent of working directory

[PhilMiller]

Changes

• There is no variable PROJECT_DIR, it seems PROJECT_BINARY_DIR was intended.

Testing

1. Run ctest in either a build tree root, or in test/ within that tree. All tests pass either way.

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• Reviewers requested with the Reviewers tool ➡️

Target Environment support

• Linux
• macOS