vim-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 vim/vim#3592)
9691f82f86

N/A patches:

vim-patch:8.1.0208: file left behind after running individual test

src/nvim/testdir/test_modeline.vim

@@ -1,7 +1,7 @@
 " 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