Extract 'Msf::Framework#threads cleaner'

MSP-11147

Extract from 'Msf::Simple::Framework' the `after(:each)` that kills and joins
threads from `framework.threads` into 'Msf::Framework#threads cleaner`.
Fix thread leaks in TaskManager spec

MSP-11147
Extract Msfcli#framework

MSP-11147

Expose Msfcli @framework as Msfcli#framework so that it can be set in
tests.  It also allows Msfcli#framework to lazily initialize and memoize
to @framework.
Remove incorrect 'Class methods' context

MSP-11147
Update Msfcli#initialize spec style

MSP-11147
Update Msfcli#usage spec style

MSP-11147
Fix context prefix

MSP-11147

Instance methods should be prefixed with `#`, not `.`.
Remove redundant 'it' in text name

MSP-11147
Update Msfclie#dump_module_list spec style

MSP-11147
Update Msfcli#usage spec style

MSP-11147
Update Msfcli#guess_payload_name spec style

MSP-11147
Update Msfcli#guess_encoder_name spec style

MSP-11147
Update Msfcli#guess_nop_name spec style

MSP-11147
Update Msfcli#generate_whitelist spec style

MSP-11147
Update Msfcli#init_modules spec style

MSP-11147
Update Msfcli#engage_mode spec style

MSP-11147
Sort context entries

MSP-11147
Fix thread-leaks in msfcli spec

MSP-11147
Use named subject instead of subject

MSP-11147
Clean up Msf::Framework spec thread-leaks

MSP-11147