docs(cdk/testing): document TestElement methods (#21103)
docs(cdk/testing): document TestElement methods (#21103)

Add missing documentation for freshly added methods: `setInputValue`, `selectOptions` & `dispatchEvent`.
andrewseguin/material2,DevVersion/material2,angular/components,angular/components,DevVersion/material2,josephperrott/material2,mmalerba/material2,josephperrott/material2,crisbeto/material2,mmalerba/material2,mmalerba/material2,andrewseguin/material2,andrewseguin/material2,mmalerba/material2,angular/components,angular/components,DevVersion/material2,crisbeto/material2,DevVersion/material2,crisbeto/material2,andrewseguin/material2,mmalerba/material2,josephperrott/material2,DevVersion/material2,crisbeto/material2,josephperrott/material2,josephperrott/material2,andrewseguin/material2