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)

src/testdir/test_recover.vim

@@ -7,7 +7,7 @@ func Test_recover_root_dir()
   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

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    600,
 /**/
     599,
 /**/