Fix clif/testing/iterator.h.
Fix clif/testing/iterator.h.

The copy assign operator should return `*this` instead of returning nothing, which causes compilation error: `non-void function does not return a value [-Werror,-Wreturn-type]`.

Set it to `default` does not fail the test cases.

PiperOrigin-RevId: 479353605

google/clif,google/clif,google/clif