test: add Leaflet adapter unit tests

[JamesLMilner]

Description of Changes

Adds unit tests for the Leaflet adapter (TerraDrawLeafletAdapter)

Link to Issue

No issue

PR Checklist

• [x] The PR title follows the conventional commit standard
• [ ] There is a associated GitHub issue
• [x] If I have added significant code changes, there are relevant tests
• [ ] If there are behaviour changes these are documented