patch 8.0.0644: the timeout for 'hlsearch' is not tested

Problem:    There is no test for 'hlsearch' timing out.
Solution:   Add a test.

src/testdir/test_hlsearch.vim

@@ -32,3 +32,21 @@ function! Test_hlsearch()
   call getchar(1)
   enew!
 endfunction
+
+func Test_hlsearch_hangs()
+  if !has('reltime') || !has('float')
+    return
+  endif
+
+  " This pattern takes forever to match, it should timeout.
+  help
+  let start = reltime()
+  set hlsearch nolazyredraw redrawtime=101
+  let @/ = '\%#=2\v(a|\1)*'
+  redraw
+  let elapsed = reltimefloat(reltime(start))
+  call assert_true(elapsed > 0.1)
+  call assert_true(elapsed < 1.0)
+  set nohlsearch redrawtime&
+  quit
+endfunc

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    644,
 /**/
     643,
 /**/