patch 8.1.2281: 'showbreak' cannot be set for one window

Problem:    'showbreak' cannot be set for one window.
Solution:   Make 'showbreak' global-local.
diff --git a/src/testdir/test_highlight.vim b/src/testdir/test_highlight.vim
index 9e1af97..83bada5 100644
--- a/src/testdir/test_highlight.vim
+++ b/src/testdir/test_highlight.vim
@@ -425,6 +425,7 @@
   let [hiCursorLine, hi_ul, hi_bg] = HiCursorLine()
 
   call NewWindow('topleft 5', 10)
+  set showbreak=xxx
   setlocal breakindent breakindentopt=min:0,shift:1 showbreak=>
   call setline(1, ' ' . repeat('a', 9) . 'bcd')
   call matchadd('Search', '\n')
@@ -482,6 +483,7 @@
 
   call CloseWindow()
   set showbreak=
+  setlocal showbreak=
   exe hiCursorLine
 endfunc