patch 8.2.2570: tests fail when run as root

Problem:    Tests fail when run as root.
Solution:   Add a comment mentioning the expected failure. (issue #7919)

src/testdir/test_edit.vim

@@ -1704,6 +1704,7 @@ func Test_edit_charconvert()
 endfunc
 
 " Test for editing a file without read permission
+" NOTE: if you run tests as root this will fail.  Don't run tests as root!
 func Test_edit_file_no_read_perm()
   CheckUnix
   call writefile(['one', 'two'], 'Xfile')