patch 8.1.1858: test for multi-byte mapping fails on some systems

Problem:    Test for multi-byte mapping fails on some systems.
Solution:   Test in another way.
diff --git a/src/testdir/test_mapping.vim b/src/testdir/test_mapping.vim
index b189296..6b9fb9c 100644
--- a/src/testdir/test_mapping.vim
+++ b/src/testdir/test_mapping.vim
@@ -241,7 +241,7 @@
 
 func Test_map_meta_multibyte()
   imap <M-á> foo
-  call assert_equal('foo', maparg("\<M-á>", 'i'))
+  call assert_match('i  <M-á>\s*foo', execute('imap'))
   iunmap <M-á>
 endfunc