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.

src/testdir/test_mapping.vim

@@ -241,7 +241,7 @@ endfunc
 
 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
 

src/version.c

@@ -769,6 +769,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1858,
 /**/
     1857,
 /**/