SDL: testaudio: Fix mouseover testing.

SDL Commits
1 
From e0b0f9a36e5b1371a9cbbf7c8debd74ad9ccfea9 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Wed, 20 Sep 2023 16:57:37 -0400
Subject: [PATCH] testaudio: Fix mouseover testing.

Not sure how this line got lost.
---
 test/testaudio.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/test/testaudio.c b/test/testaudio.c
index bd158383119d..94de784715d3 100644
--- a/test/testaudio.c
+++ b/test/testaudio.c
@@ -212,6 +212,8 @@ static Thing *UpdateMouseOver(const float x, const float y)
         thing = FindThingAtPoint(x, y);
     }
 
+    mouseover_thing = thing;
+
     if (!thing) {
         SetDefaultTitleBar();
     } else if (thing->titlebar) {