patch 8.1.1488: summary of tests has incorrect failed count

Problem:    Summary of tests has incorrect failed count.
Solution:   Add to the failed count instead of setting it. (Christian Brabandt)

src/testdir/summarize.vim

@@ -6,7 +6,7 @@ if 1
     if a:type ==# 'executed'
       let g:executed += (a:match+0)
     elseif a:type ==# 'failed'
-      let g:failed = a:match+0
+      let g:failed += a:match+0
     elseif a:type ==# 'skipped'
       let g:skipped += 1
       call extend(g:skipped_output, ["\t".a:match])

src/version.c

@@ -767,6 +767,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1488,
 /**/
     1487,
 /**/