ip6 tests: Add very simplistic extension header tests

Add a simple test case which can exercise some of the IPv6 extension
header code paths. At the moment only a small set of extension headers
is implemented and no options to the ones which take them.
Also implements a "bad" case to make sure that error handling works.

The tests were used to test m_pullup() changes to the code paths while
removing the KAME PULLDOWN_TEST cases and related macros.

MFC after: 3 weeks
Sponsored by: Netflix