MSP-11605
The 'Msf::Framework#threads cleaner' shared context fails with a
RuntimeError if `framework.threads?` is false, which would indicate that
cleaning is unnecessary. This change stops 'Msf::Framework#threads
cleaner' from accessing `framework.threads`, which would create threads
only to immediately clean them up.
MSP-11147
Extract from 'Msf::Simple::Framework' the `after(:each)` that kills and joins
threads from `framework.threads` into 'Msf::Framework#threads cleaner`.
Luke Imhoff