test(bezier-react): add test to useIsMounted (channel-io#1776)

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## Related Issue
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## Summary
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add test to useIsMounted

## Details
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<img width="649" alt="image"
src="https://github.com/channel-io/bezier-react/assets/102217654/d43094be-af70-4066-a2ee-f9e871f8c2da">

### Breaking change? (Yes/No)
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---------

Co-authored-by: Yang Wooseong (Andrew) <[email protected]>

packages/bezier-react/src/hooks/useIsMounted.test.ts

@@ -0,0 +1,11 @@
+import { renderHook } from '@testing-library/react'
+
+import useIsMounted from './useIsMounted'
+
+describe('useIsMounted', () => {
+  it('should return true when mounted', () => {
+    const { result } = renderHook(() => useIsMounted())
+
+    expect(result.current).toBe(true)
+  })
+})