patch 9.0.0363: common names in test files causes tests to be flaky

Problem:    Common names in test files causes tests to be flaky.
Solution:   Use more specific names.
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index 737dd0b..0072e3d 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -3062,7 +3062,7 @@
   augroup END
   let save_cpo = &cpo
   set cpo+=f
-  call assert_fails('r Xfile', ['E812:', 'E484:'])
+  call assert_fails('r Xchangebuf', ['E812:', 'E484:'])
   call assert_equal('somefile', @%)
   let &cpo = save_cpo
   augroup TestAuCmd