chore(deps): update dependency @web/test-runner to v0.20.0 #4314

Job	Run time
Verify changes (lit-element)	14s
Verify changes (lit-element-ts-esbuild)	15s
Verify changes (lit-element-ts-tsc)	15s
Verify changes (html-test)	15s
Verify changes (preact-htm)	15s
Verify changes (preact-jsx)	14s
Verify changes (preact-tsx)	14s
Verify changes (puppeteer)	13s
Verify changes (playwright)	12s
Verify changes (import-maps)	13s
Verify changes (mock-es-module)	14s
Verify changes (visual-regression)	1s
Verify changes (saucelabs)	11s
Verify changes (guides/test-runner/getting-started)	17s
Verify changes (guides/test-runner/watch-and-debug)	13s
Verify changes (guides/test-runner/playwright)	14s
Verify changes (guides/test-runner/responsive)	14s
Verify changes (guides/test-runner/typescript)	15s
Verify changes (react-htm)	13s
Verify changes (react-jsx)	13s
Verify changes (react-tsx)	12s
Verify changes (snowpack-lit)	1s
Verify changes (snowpack-react)	1s
Verify changes (snowpack-svelte)	1s
	4m 40s

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