Updating split validator to allow duplicates across test sets

Changelogs

Updating the split field validator in the BenchmarkSpecification model to check for duplicates within individual test sets rather than across all test sets
Added test to ensure duplicates in a nested test set are caught
Added test to ensure that duplicate indices are allowed across indices

Checklist:

[x] Was this PR discussed in an issue? It is recommended to first discuss a new feature into a GitHub issue before opening a PR. https://github.com/polaris-hub/polaris/issues/157
[x] Add tests to cover the fixed bug(s) or the newly introduced feature(s) (if appropriate).
[x] Update the API documentation if a new function is added, or an existing one is deleted.
[x] Write concise and explanatory changelogs above.
[x] If possible, assign one of the following labels to the PR: feature, fix, chore, documentation or test (or ask a maintainer to do it for you).

discussion related to that PR