vim-patch:8.0.1257: no test for fix of undefined behavior

Problem:    No test for fix of undefined behavior.
Solution:   Add a test. (closes vim/vim#2255)
2973daafe1

src/nvim/testdir/test_search.vim

@@ -468,3 +468,7 @@ func Test_search_undefined_behaviour()
   call term_sendkeys(g:buf, ":qa!\<cr>")
   bwipe!
 endfunc
+
+func Test_search_undefined_behaviour2()
+  call search("\%UC0000000")
+endfunc