test: add unit tests for XID filtering logic

Add comprehensive unit test coverage for XID filtering:

Test Coverage:
- XID parsing logic (newHealthCheckXIDs) - 10 test cases
- XID filtering with environment variables - 5 test cases
- Default ignored XIDs validation
- Environment variable override behavior

Key Features:
- Tests XID filtering (13, 31, 43, 45, 68, 109 filtered by default)
- Validates 'all' and 'xids' keywords
- Verifies enabled overrides disabled
- All tests pass with -race flag

Inspired by elezar/refactor-health (dab53b9).

Signed-off-by: Carlos Eduardo Arango Gutierrez <eduardoa@nvidia.com>

Author: ArangoGutierrez