patch 8.0.1224: still interference between test functions

Problem:    Still interference between test functions.
Solution:   Clear autocommands. Wipe all buffers.  Fix tests that depend on a
            specific start context.
diff --git a/src/testdir/test_packadd.vim b/src/testdir/test_packadd.vim
index 66cbb47..c83d4a8 100644
--- a/src/testdir/test_packadd.vim
+++ b/src/testdir/test_packadd.vim
@@ -2,7 +2,7 @@
 
 
 func SetUp()
-  let s:topdir = expand('%:h') . '/Xdir'
+  let s:topdir = getcwd() . '/Xdir'
   exe 'set packpath=' . s:topdir
   let s:plugdir = s:topdir . '/pack/mine/opt/mytest'
 endfunc