Rename temporary user data directory

The default name prefix for the temporary user data dir is `d` which is
not very helpful when trying to figure out where the directory comes
from. The patch makes the directory name more explicit.

Author: sigmike

lib/ferrum/browser/process.rb

@@ -67,7 +67,7 @@ def initialize(options)
         @logger = options[:logger]
         @process_timeout = options.fetch(:process_timeout, PROCESS_TIMEOUT)
 
-        tmpdir = Dir.mktmpdir
+        tmpdir = Dir.mktmpdir("ferrum_user_data_dir_")
         ObjectSpace.define_finalizer(self, self.class.directory_remover(tmpdir))
         @user_data_dir = tmpdir
         @command = Command.build(options, tmpdir)