patch 8.0.0645: no error for illegal back reference in NFA engine

Problem:    The new regexp engine does not give an error for using a back
            reference where it is not allowed. (Dominique Pelle)
Solution:   Check the back reference like the old engine. (closes #1774)

src/testdir/test_hlsearch.vim

@@ -38,11 +38,11 @@ func Test_hlsearch_hangs()
     return
   endif
 
-  " This pattern takes forever to match, it should timeout.
+  " This pattern takes a long time to match, it should timeout.
   help
   let start = reltime()
   set hlsearch nolazyredraw redrawtime=101
-  let @/ = '\%#=2\v(a|\1)*'
+  let @/ = '\%#=1a*.*X\@<=b*'
   redraw
   let elapsed = reltimefloat(reltime(start))
   call assert_true(elapsed > 0.1)

src/testdir/test_regexp_latin.vim

@@ -62,3 +62,13 @@ func Test_eow_with_optional()
     call assert_equal(expected, actual)
   endfor
 endfunc
+
+func Test_backref()
+  new
+  call setline(1, ['one', 'two', 'three', 'four', 'five'])
+  call assert_equal(3, search('\%#=1\(e\)\1'))
+  call assert_equal(3, search('\%#=2\(e\)\1'))
+  call assert_fails('call search("\\%#=1\\(e\\1\\)")', 'E65:')
+  call assert_fails('call search("\\%#=2\\(e\\1\\)")', 'E65:')
+  bwipe!
+endfunc

src/testdir/test_statusline.vim

@@ -223,7 +223,7 @@ func Test_statusline()
   set statusline=ab%(cd%q%)de
   call assert_match('^abde\s*$', s:get_statusline())
   copen
-  call assert_match('^abcd\[Quickfix List\1]de\s*$', s:get_statusline())
+  call assert_match('^abcd\[Quickfix List]de\s*$', s:get_statusline())
   cclose
 
   " %#: Set highlight group. The name must follow and then a # again.