Tell use to run with `rake spec` to see Thread.new caller

MSP-11147

If the log isn't available, tell the user to rerun with `rake spec`
instead of printing nothing after the `:\n`, which looks incomplete.

lib/metasploit/framework/spec/threads/suite.rb

@@ -34,6 +34,8 @@ module Metasploit
                     # LOG_PATHNAME may not exist if suite run without `rake spec`
                     if LOG_PATHNAME.exist?
                       log = LOG_PATHNAME.read()
+                    else
+                      log "Run `rake spec` to log where Thread.new is called."
                     end
 
                     raise RuntimeError,