FIX CI: Remove Directory.Build.props to fix xunit issue

The root cause of all CI failures is Directory.Build.props which forces
both projects to share the same obj directory and project.assets.json file.

When building, the main project restore overwrites the assets file and
removes xunit dependencies needed by the test project.

Solution: Remove Directory.Build.props. Let each project have its own
obj and bin directories. This prevents conflicts.

Tested locally - all 92 tests pass.

Author: Alex J Lennon