patch 8.2.4180: 'balloonexpr' is evaluated in the current script context

Problem:    'balloonexpr' is evaluated in the current script context.
Solution:   Use the script context where the option was set.
diff --git a/src/testdir/test_balloon.vim b/src/testdir/test_balloon.vim
index 319e546..ed0c6c1 100644
--- a/src/testdir/test_balloon.vim
+++ b/src/testdir/test_balloon.vim
@@ -10,7 +10,8 @@
 
 let s:common_script =<< trim [CODE]
   call setline(1, ["one one one", "two tXo two", "three three three"])
-  set balloonevalterm balloonexpr=MyBalloonExpr() balloondelay=100
+  set balloonevalterm balloonexpr=MyBalloonExpr()..s:trailing balloondelay=100
+  let s:trailing = '<'  " check that script context is set
   func MyBalloonExpr()
     return "line " .. v:beval_lnum .. " column " .. v:beval_col .. ":\n" .. v:beval_text
   endfun