[EuiDualRange] Add regression test for Kibana use of ref prop

[cee-chen]

Summary

Whether we like it or not, there are currently 2 instances of Kibana using the ref prop on EuiDualRange to call the class onResize method: elastic/kibana@5c575f9. I discovered this because the typing for EuiDualRange's ref prop became extremely annoying once we started wrapping in withEuiTheme.

The ideal solution, converting EuiDualRange from a class component to a function component, unfortunately also takes away Kibana's ability to call onResize. This means if/when we do convert EuiDualRange to a function component, we'll need to deliberately expose onResize via React's useImperativeHandle API.

To ensure we don't forget this, I've added a unit test for calling rangeRef.current.onResize() that will fail if we perform the conversion without adding the ref method.

QA

General checklist

N/A, dev/test only

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_6467/

[1Copenut]

👍 LGTM! Your comments and logic make this very easy to follow as a defensive test for future breaking changes.