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_ins_complete.vim b/src/testdir/test_ins_complete.vim
index 2dd2fb7..bdad26a 100644
--- a/src/testdir/test_ins_complete.vim
+++ b/src/testdir/test_ins_complete.vim
@@ -1,6 +1,7 @@
 
 " Test for insert expansion
 func Test_ins_complete()
+  edit test_ins_complete.vim
   set ff=unix
   call writefile(["test11\t36Gepeto\t/Tag/",
 	      \ "asd\ttest11file\t36G",