Test "completion widget" wrapping

[danielshahaf]

7e7e291 broke completion widgets (see #278 (comment); note preceding comments on that issue refer to the issue now tracked as #280).

The test suite should be extended to cover the "completion widget" wrapping codepath.

[danielshahaf]

Another variant: on #356, before zmodload zsh/zle was added to the tests, they silently fell back to the zsh≤5.2 codepath.