patch 8.2.1183: assert_fails() checks the last error message

Problem:    assert_fails() checks the last error message.
Solution:   Check the first error, it is more relevant.  Fix all the tests
            that rely on the old behavior.
diff --git a/src/testdir/test_cpoptions.vim b/src/testdir/test_cpoptions.vim
index ce4e00d..02cadf4 100644
--- a/src/testdir/test_cpoptions.vim
+++ b/src/testdir/test_cpoptions.vim
@@ -104,7 +104,7 @@
   source Xfile
   call assert_equal([1, 2], g:l)
   set cpo+=C
-  call assert_fails('source Xfile', 'E10:')
+  call assert_fails('source Xfile', ['E697:', 'E10:'])
   call delete('Xfile')
   let &cpo = save_cpo
 endfunc