chore: deduplicate Preact during testing
chore: deduplicate Preact during testing

This allows us to safely link other preact using libraries
into this one.

Note that de-duplication happens naturally on install of
non-linked dependencies.

Closes https://github.com/bpmn-io/bpmn-properties-panel/issues/27

bpmn-io/bpmn-js-properties-panel,bpmn-io/bpmn-js-properties-panel