tests: Extend aspath_test.c with cases for invalid segments & attributes

* aspath_test.c: Add more test cases. In particular ones to cover the
  last invalid-segment problem. Also add ability to specify aspath attribute
  headers and test them somewhat.

  NB: It's obvious this test has not been run for a year by anyone, despite
  2 non-trivial commits to bgpd aspath code.
1 file changed