[6.x] Add waitForInput

[SjorsO]

This PR adds a waitForInput method that waits for an input field by name. This PR is similar to #914.

I have some flaky tests in CI that very sometimes fail to type in a text input. The tests look something like this:

->clickAndWaitForReload('@edit-'.$user->id)
->type('name', 'Sjors')

The new waitForInput method can solve this:

->clickAndWaitForReload('@edit-'.$user->id)
->waitForInput('name')
->type('name', 'Sjors')

I haven't added any tests. I still can't figure out how to craft a mock that calls itself.