Reduce noise in unit test logs

The logs contain a number of the following type of error:
```
      Error: Not implemented: window.computedStyle(elt, pseudoElt)
          at module.exports
(/home/prow/go/src/github.com/tektoncd/dashboard/node_modules/jsdom/lib/jsdom/browser/not-implemented.js:9:17)
          at Window.getComputedStyle
(/home/prow/go/src/github.com/tektoncd/dashboard/node_modules/jsdom/lib/jsdom/browser/Window.js:798:7)
          at getComputedStyle
(/home/prow/go/src/github.com/tektoncd/dashboard/node_modules/carbon-components-react/lib/components/Tooltip/Tooltip.js:81:55)
```

This is caused by the Carbon Tooltip component passing a second argument to
`getComputedStyle` referencing a pseudo element. This is not implemented in
jsdom resulting in the errors.

Since this shouldn't affect the tests, strip the second argument when calling
`getComputedStyle` in the tests to resolve the error and reduce noise in the logs.

config_frontend/setupTests.js

@@ -32,6 +32,9 @@ window.HTMLElement.prototype.scrollIntoView =
 window.TextDecoder = TextDecoder;
 window.TextEncoder = TextEncoder;
 
+const { getComputedStyle } = window;
+window.getComputedStyle = element => getComputedStyle(element);
+
 Object.defineProperty(window, 'matchMedia', {
   writable: true,
   value: jest.fn().mockImplementation(query => ({