Include testing for Resultset objects and related utils

[betochimas]

While Resultset, introduced in #3705 , will improve the testing for cuGraph developers, breaking changes and bugs caused by Resultset are observed only by a failure from python tests within the traversal module. Coverage is needed to properly test Resultset, similar to how the Dataset API is tested within test_datasets.py.

This issue serves as a roadmap for fully incorporating Resultset into cuGraph's testing, along with suggested improvements for Resultset and Dataset, which are expected to have more in common in the future.

Tasks

Beta Give feedback

1. Create test_resultset.py to add proper coverage to Resultset, load_resultset, get_resultset
2. Refactor tests from centrality module to use Resultset objects and remove the nx dependency
3. Refactor tests from community module to use Resultset objects and remove the nx dependency
4. Refactor tests from components module to use Resultset objects and remove the nx dependency
5. Refactor tests from core module to use Resultset objects and remove the nx dependency
6. Refactor tests from layout module to use Resultset objects and remove the nx dependency
7. Refactor tests from link_analysis module to use Resultset objects and remove the nx dependency
8. Refactor tests from link_prediction module to use Resultset objects and remove the nx dependency
9. Refactor tests from nx module to use Resultset objects and remove the nx dependency
10. Refactor tests from sampling module to use Resultset objects and remove the nx dependency
11. Refactor tests from structure module to use Resultset objects and remove the nx dependency
12. Refactor tests from tree module to use Resultset objects and remove the nx dependency
13. Refactor the Resultset and Dataset APIs to reuse code patterns and have them share a location
14. Extend Dataset.get_graph to allow different source and destination column names other than 'src' and 'dst'