Tests: Improve string cast render test

[aduth]

Raised at: #4879 (comment) (props @johnbillion)

This pull request seeks to improve dynamic block rendering tests which verify the string output of a render call. While the behavior itself is working as expected, the tests will wrongly pass even if the string cast was not implemented.

Testing instructions:

Ensure unit tests pass:

npm run test-unit-php

Extra credit: Ensure unit tests fail when removing (string) cast:

gutenberg/lib/class-wp-block-type.php

Line 116 in 967d010

return (string) call_user_func( $this->render_callback, $attributes );

[gziolo]

LGTM 👍