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