patch 8.1.0844: when timer fails test will hang forever

Problem:    When timer fails test will hang forever.
Solution:   Use reltime() to limit waiting time. (Ozaki Kiichi, closes #3878)

src/testdir/test_timers.vim

@@ -257,9 +257,11 @@ func Test_getchar_zero()
     return
   endif
 
+  " Measure the elapsed time to avoid a hang when it fails.
+  let start = reltime()
   let id = timer_start(20, {id -> feedkeys('x', 'L')})
   let c = 0
-  while c == 0
+  while c == 0 && reltimefloat(reltime(start)) < 0.2
     let c = getchar(0)
     sleep 10m
   endwhile

src/version.c

@@ -783,6 +783,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    844,
 /**/
     843,
 /**/