bug#34506: 27.0.50: push-button bug with basic text-property button

[Eli Zaretskii]

> From: "Basil L. Contovounesios" <contovob@tcd.ie>
> Cc: Eli Zaretskii <eliz@gnu.org>, 34506@debbugs.gnu.org
> Date: Mon, 25 Feb 2019 02:40:26 +0000
> 
> > If buttons and widgets are wholly incompatible and separate then button-at
> > should return nil when on a widget even if both buttons and widgets use the 
> > same
> > underlying mechanism.
> 
> How's the following?

LGTM, but can we please have a simple test for this, so that this
never again regresses?

Thanks.