refactor: imports shared polyfills in setupTests (#6571)

src/setupTests.ts

 import '@testing-library/jest-dom' // jest custom assertions
 import 'polyfills'
 import 'jest-styled-components' // adds style diffs to snapshot tests
+import 'polyfills'
 
 import type { createPopper } from '@popperjs/core'
 import { useWeb3React } from '@web3-react/core'
@@ -32,8 +33,6 @@ global.matchMedia =
     }
   })
 
-global.ResizeObserver = ResizeObserver
-
 jest.mock('@popperjs/core', () => {
   const core = jest.requireActual('@popperjs/core')
   return {