Add callback for intermittent tests

[santib]

Hi, I'm opening the PR just in case you are interested in having a mechanism to notify about intermittent tests. What this code does is call the RSpec.configuration.intermittent_callback callback if a test passes while retrying it. That way, we can keep a list of the intermittent tests in the project and fix them.

Example callback:

  config.intermittent_callback = proc do |ex|
    Slack::Reporter.new(ex).post_to_channel
  end

[santib]

any update on this @rekahsoft ?