patch 7.4.1844

Problem:    Using old function name in comment.  More functions should start
            with test_.
Solution:   Rename function in comment. (Higashi Higashi) Rename
            disable_char_avail_for_testing() to test_disable_char_avail().
            And alloc_fail() to test_alloc_fail().

src/version.c

@@ -753,6 +753,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1844,
 /**/
     1843,
 /**/