patch 8.0.1417: test doesn't search for a sentence

Problem:    Test doesn't search for a sentence. Still fails when searching for
            start of sentence. (Dominique Pelle)
Solution:   Add paren. Check for MAXCOL in dec().
diff --git a/src/testdir/test_search.vim b/src/testdir/test_search.vim
index 84edfbb..9b96b49 100644
--- a/src/testdir/test_search.vim
+++ b/src/testdir/test_search.vim
@@ -733,6 +733,8 @@
 func Test_search_sentence()
   new
   " this used to cause a crash
-  call assert_fails("/\\%'", 'E486')
+  call assert_fails("/\\%')", 'E486')
   call assert_fails("/", 'E486')
+  /\%'(
+  /
 endfunc