patch 8.2.1432: various inconsistencies in test files

Problem:    Various inconsistencies in test files.
Solution:   Add modelines where they were missing.  Use Check commands instead
            of silently skipping over tests.  Adjust indents and comments.
            (Ken Takata, closes #6695)
diff --git a/src/testdir/test_packadd.vim b/src/testdir/test_packadd.vim
index fcb8b80..d7a38da 100644
--- a/src/testdir/test_packadd.vim
+++ b/src/testdir/test_packadd.vim
@@ -1,5 +1,6 @@
 " Tests for 'packpath' and :packadd
 
+source check.vim
 
 func SetUp()
   let s:topdir = getcwd() . '/Xdir'
@@ -115,9 +116,7 @@
 endfunc
 
 func Test_packadd_symlink_dir()
-  if !has('unix')
-    return
-  endif
+  CheckUnix
   let top2_dir = s:topdir . '/Xdir2'
   let real_dir = s:topdir . '/Xsym'
   call mkdir(real_dir, 'p')
@@ -150,9 +149,7 @@
 endfunc
 
 func Test_packadd_symlink_dir2()
-  if !has('unix')
-    return
-  endif
+  CheckUnix
   let top2_dir = s:topdir . '/Xdir2'
   let real_dir = s:topdir . '/Xsym/pack'
   call mkdir(top2_dir, 'p')