patch 8.0.0600: test_recover fails on some systems

Problem:    test_recover fails on some systems.
Solution:   Explicitly check if "/" is writable. (Ken Takata)
diff --git a/src/testdir/test_recover.vim b/src/testdir/test_recover.vim
index 33cd54b..2e34985 100644
--- a/src/testdir/test_recover.vim
+++ b/src/testdir/test_recover.vim
@@ -7,7 +7,7 @@
   call assert_fails('recover', 'E305:')
   close!
 
-  if has('win32')
+  if has('win32') || filewritable('/') == 2
     " can write in / directory on MS-Windows
     set dir=/notexist/
   endif