[FR] Create Unit Test for non-ecs-schema.json

**Is your feature request related to a problem? Please describe.**
https://github.com/elastic/detection-rules/pull/2319
https://github.com/elastic/detection-rules/pull/2319#pullrequestreview-1116099473

**Describe the solution you'd like**
- Unit test to check for duplicates
- Check if the field is in the schema
- Potential additional checks that are difficult to do since some fields are used in old rules in older branches
- Check if the field is in use in rules
- Check for bad keywords in the PR