security/py-obfsproxy-tor: Fix runtime dependency error

• Change runtime dependency entry point Twisted_Core back to Twisted
• Update checksum for CHEESESHOP
• Add test target

Reviewed by: cs, koobs
Approved by: cs (maintainer), koobs (ports)
Differential Revision: https://reviews.freebsd.org/D13452
MFH: 2018Q2