Refactor tests/sys/opencrypto/runtests

Refactor tests/sys/opencrypto/runtests

* Convert from plain to TAP for slightly improved introspection when skipping
  the tests due to requirements not being met.
* Test for the net/py-dpkt (origin) package being required when running the
  tests, instead of relying on a copy of the dpkt.py module from 2014. This
  enables the tests to work with py3. Subsequently, remove
  `tests/sys/opencrypto/dpkt.py(c)?` via `make delete-old`.
* Parameterize out `python2` as `$PYTHON`.

PR:		237403
MFC after:	1 week