Statistics more unit tests

[MarkBaker]

This is:

- [ ] a bugfix
- [ ] a new feature
- [X] unit tests

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

Unit tests for SKEW() and TRIMMEAN() functions