patch 8.1.0088: terminal test for stdout and stderr is a bit flaky

Problem:    Terminal test for stdout and stderr is a bit flaky.
Solution:   Wait for both stdout and stderr to have been processed. (Ozaki
            Kiichi, closes #2991)

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    88,
 /**/
     87,
 /**/