patch 8.0.0437: packadd test does not fully work

Problem:    The packadd test does not create the symlink correctly and does
            not test the right thing.
Solution:   Create the directory and symlink correctly.

src/testdir/test_packadd.vim

@@ -75,7 +75,8 @@ func Test_packadd_symlink_dir()
   endif
   let top2_dir = s:topdir . '/Xdir2'
   let real_dir = s:topdir . '/Xsym'
-  exec "silent !ln -s" real_dir top2_dir
+  call mkdir(real_dir, 'p')
+  exec "silent !ln -s Xsym"  top2_dir
   let &rtp = top2_dir . ',' . top2_dir . '/after'
   let &packpath = &rtp
 

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    437,
 /**/
     436,
 /**/