Bug 1600191 - Make test_bug159346.xhtml specify `min-height` and `min-width` of scrollbar button to avoid native theme difference r=NeilDeakin

test_bug159346.xhtml modifies UA stylesheet to show scrollbar button.
However, on Ubuntu 18.04, its width is 0.  Therefore, the following
script fails to test click on the button.

This patch specifies its `min-width` and `min-height` to make it
clickable on any platforms.

Differential Revision: https://phabricator.services.mozilla.com/D58189

--HG--
extra : moz-landing-system : lando

layout/xul/test/test_bug159346.xhtml

@@ -20,7 +20,7 @@ var scrollbar = document.getElementById("scrollbar");
 var downButton;
 
 var domWinUtils = SpecialPowers.DOMWindowUtils;
-  domWinUtils.loadSheetUsingURIString('data:text/css,@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); scrollbarbutton[type="increment"][sbattr="scrollbar-down-bottom"] { display: -moz-box; }', domWinUtils.AGENT_SHEET);
+domWinUtils.loadSheetUsingURIString('data:text/css,@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); scrollbarbutton[type="increment"][sbattr="scrollbar-down-bottom"] { display: -moz-box; min-width: 3px; min-height: 3px; }', domWinUtils.AGENT_SHEET);
 
 function init()
 {