tests: Fix testbgpmpattr and make check, broken by BGP NHT.

* bgp_mp_attr_test.c: Fix segfault due to uninitialised bgp_nexthop_cache
  globals in nexthop parsing path, which require bgp_scan_init() to have
  been called.

  Fixes issue introduced with:

  'bgpd, zebra: Use next hop tracking for connected routes too'

tests/bgp_mp_attr_test.c

@@ -758,6 +758,7 @@ main (void)
   bgp_option_set (BGP_OPT_NO_LISTEN);
   bgp_attr_init ();
   bgp_address_init ();
+  bgp_scan_init ();
   
   if (fileno (stdout) >= 0) 
     tty = isatty (fileno (stdout));