How to test Material-UI Checkbox is checked with react-testing-library?

Since the checkbox is rendering an input I would work with it rather than focusing on the image.

You could do something like this:

const checkbox = getByTestId('checkbox-1234').querySelector('input[type="checkbox"]')
expect(checkbox).toHaveProperty('checked', true)

The easiest way I have found to locate a checkbox is by adding a label to it

<FormControlLabel
  htmlFor="first-checkBox"
  label="First Checkbox"
  control={ <Checkbox
        checked={checked}
        onChange={handleChange}
        inputProps={{ 'aria-label': 'primary checkbox' }}
      />
  }
/>

later in test case

const checkboxEl = screen.getByLabelText('First Checkbox') as HTMLInputElement
expect(checkboxEl).toBeChecked()

if the test fails because doesn't recognize toBeChecked as part of the available assertions is because you need to import

import '@testing-library/jest-dom/extend-expect'