Update tests in navigator_spec.js

What does this MR do and why?

Update tests in file navigator_spec.js, removed toBeFalsy matchers and replaced with more specific matchers.

For issue #369728 (closed)

Screenshots or screen recordings

n/a

How to set up and validate locally

yarn jest spec/frontend/ide/components/preview/navigator_spec.js

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.