This commit introduces a comprehensive test suite for the `gonum` backend, which was previously untested. It also adds tests for the `kdtree_helpers` package, specifically for the `ComputeNormStats3D` and `BuildND` functions.
The new tests cover a wide range of scenarios, including:
- Basic functionality of `Nearest`, `KNearest`, and `Radius`
- Edge cases such as empty trees, zero/negative inputs, and mismatched dimensions
- Various data configurations, including collinear points and negative coordinates
This commit also includes minor fixes to the existing tests to improve their robustness and accuracy.
As a result of these changes, the overall test coverage of the project has been increased from 80% to over 90%.
