Provides a test backend (aka test mode)

[robacarp]

fixes #43

Two approaches were suggested in the issue:

• Run jobs synchronously.
• Catalog jobs that were enqueued, and allow the history to be checked.

The synchronous job pattern is used elsewhere and provides convenience for integration testing. A developer can then test "a user clicks this button and an email is sent to them". However it also adds code to the worker and runner -- something I try to avoid if at all possible to keep mosquito as fast as possible.

I've chosen to implement a testing backend which can be activated in the testing setup code for a project and all enqueues will simply get stored for later inspection / assertion. This avoids adding test_mode=true? checks to worker code. For situations where you need to test the affects of a job, use #run as usual.

It'll look like this:

Mosquito.configure do |settings|
  settings.backend = Mosquito::TestBackend
end

Then in your tests:

describe "testing" do
  it "enqueues the job" do
    # build and enqueue a job
    job_run = EchoJob.new(text: "hello world").enqueue

    # assert that the job was enqueued
    lastest_enqueued_job = Mosquito::TestBackend.enqueued_jobs.last

    # check the job config
    assert_equal "hello world", latest_enqueued_job.config["text"]

    # check the job_id matches
    assert_equal job_run.id, latest_enqueued_job.id

    # optionally, truncate the history
    Mosquito::TestBackend.flush_enqueued_jobs!
  end
end

This is a bare bones test helper, but I think it'll save me and other folks some headache.

[jwoertink]

Looks pretty straight forward. I dig it.