Use before(:each) instead of :all

Fixes deprecation warnings in newer rspec like these

  WARNING: subject accessed in a `before(:all)` hook at:
    /metasploit-framework/spec/lib/rex/post/meterpreter/packet_spec.rb:455:in `block (3 levels) in <top (required)>'

  This is deprecated behavior that will not be supported in RSpec 3.

Also switches to using named subjects for easier reading.