Add comprehensive unit tests for MergeUncoreGroups function

This commit adds unit tests for the MergeUncoreGroups function that was
recently modified to eliminate duplicate uncore events across groups
(commit cd5bcf2). The duplicate event elimination prevents perf from
misreporting event values when the same event appears in multiple groups.

Test coverage includes:
- Duplicate event elimination across 2 and 3 groups
- Verification that first occurrence is preserved
- Group merging behavior after deduplication
- Preservation of metric names during merge
- Edge cases: empty input, single group, all empty events
- Validation that empty events don't count as duplicates
- Incompatible unit handling

All tests follow the project's established patterns using table-driven
tests and the testify assertion library.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>
Signed-off-by: Harper, Jason M <[email protected]>

Author: harp-intel