patch 8.1.0506: modeline test fails when run by root

Problem:    Modeline test fails when run by root.
Solution:   Set 'modeline' for the test. (James McCoy, closes #3592)

src/testdir/test_modeline.vim

@@ -1,9 +1,12 @@
 " Tests for parsing the modeline.
 
 func Test_modeline_invalid()
-  " This was reading before allocated memory.
+  " This was reading allocated memory in the past.
   call writefile(['vi:0', 'nothing'], 'Xmodeline')
+  let modeline = &modeline
+  set modeline
   call assert_fails('split Xmodeline', 'E518:')
+  let &modeline = modeline
   bwipe!
   call delete('Xmodeline')
 endfunc

src/version.c

@@ -792,6 +792,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    506,
 /**/
     505,
 /**/