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_writefile.vim

@@ -406,6 +406,7 @@ func Test_patchmode()
 endfunc
 
 " Test for writing to a file in a readonly directory
+" NOTE: if you run tests as root this will fail.  Don't run tests as root!
 func Test_write_readonly_dir()
   " On MS-Windows, modifying files in a read-only directory is allowed.
   CheckUnix