test: add extra unit test to openlayers.adapter.spec

[AnneFiger]

Description of Changes

Add in a test for getLngLatFromEvent method in open layers adaptor Slightly change the way we pass in the mock map in the adaptor for testing so this is easier to reuse

Link to Issue

https://github.com/JamesLMilner/terra-draw/issues/90

PR Checklist

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