Randomize the honeypot hidden field title

Problem/Motivation

The honeypot hidden field has a fixed title "Leave this field blank" which make it recognizable by bots.

Proposed resolution

Make the title more dynamic (even randomize it)

* Option 1: add:

function _honeypot_get_field_title() {
  $operations = ['+', '-', '*', '/'];
  $output = implode(' ', [rand(1,99), $operations[rand(0,3)], rand(1,99), '= ?']);

  return $output; // Example output: 4 * 57 = ?
}

* Option 2: use Random::name

* Option 3: use Random::word

Remaining tasks

Per #3154557-30: Randomize the honeypot hidden field title → :

1. Write functional test cases: "in general, new features or changed features should have a test case - that's really the only way we can maintain this module in the long run."
2. Label should be configurable: "And since there are so many users, it would be great if this label were configurable rather than just changing the way it works. It could use a simple setting variable with a hook_update_N() to initialize the new setting for existing sites (and to ensure that the "old" string was still used for existing sites by default) and a small test case to check that the setting does the right thing."
3. Refactor new function into config: Using Random::word() seems like a good alternative strategy for those who want it. But not in a function beginning with "_" in the .module file - this should be a config setting in HoneypotSettingsForm and accessed from the config in .module like we do with other settings.