patch 8.1.2336: when an expr mapping moves the cursor it is not restored

Problem:    When an expr mapping moves the cursor it is not restored.
Solution:   Position the cursor after an expr mapping. (closes #5256)

src/testdir/test_mapping.vim

@@ -2,6 +2,7 @@
 
 source shared.vim
 source check.vim
+source screendump.vim
 
 func Test_abbreviation()
   " abbreviation with 0x80 should work
@@ -461,3 +462,33 @@ func Test_list_mappings()
   iunmap <S-/>
   call assert_equal(['No mapping found'], execute('imap')->trim()->split("\n"))
 endfunc
+
+func Test_expr_map_restore_cursor()
+  CheckScreendump
+
+  let lines =<< trim END
+      call setline(1, ['one', 'two', 'three'])
+      2
+      set ls=2
+      hi! link StatusLine ErrorMsg
+      noremap <expr> <C-B> Func()
+      func Func()
+	  let g:on = !get(g:, 'on', 0)
+	  redraws
+	  return ''
+      endfunc
+      func Status()
+	  return get(g:, 'on', 0) ? '[on]' : ''
+      endfunc
+      set stl=%{Status()}
+  END
+  call writefile(lines, 'XtestExprMap')
+  let buf = RunVimInTerminal('-S XtestExprMap', #{rows: 10})
+  call term_wait(buf)
+  call term_sendkeys(buf, "\<C-B>")
+  call VerifyScreenDump(buf, 'Test_map_expr_1', {})
+
+  " clean up
+  call StopVimInTerminal(buf)
+  call delete('XtestExprMap')
+endfunc