patch 9.1.0810: cannot easily adjust the |:find| command

Problem:  cannot easily adjust the |:find| command
Solution: Add support for the 'findexpr' option (Yegappan Lakshmanan)

closes: #15901
closes: #15905

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_options.vim b/src/testdir/test_options.vim
index 73e6d85..35b7d48 100644
--- a/src/testdir/test_options.vim
+++ b/src/testdir/test_options.vim
@@ -1570,7 +1570,7 @@
 
 " Test for changing options in a sandbox
 func Test_opt_sandbox()
-  for opt in ['backupdir', 'cdpath', 'exrc']
+  for opt in ['backupdir', 'cdpath', 'exrc', 'findexpr']
     call assert_fails('sandbox set ' .. opt .. '?', 'E48:')
     call assert_fails('sandbox let &' .. opt .. ' = 1', 'E48:')
   endfor