Add top level rake task to run all tests
fix for payments made via /admin

orders placed via /admin were being charged twice.

See these two lines from Spree::Admin::PaymentsController:

    while @order.next; end
    @payment.process! if @order.completed?

For incomplete orders the first line will trigger callbacks that cause
the payment to be charged if the order reaches the "completed" state.
Then the second line will process the payment *again*, resulting in a
double charge.

We can't just remove the second line because orders that are already
complete won't trigger any callbacks from the first line, so these
orders need to have their payments charged manually.

The effect of the bug in our case was that Braintree rejected the second
(duplicate) billing on the payment, and this resulted in the Payment
object being marked as failed when actually it succeeded on the first
attempt. So it appeared from within Spree that a successful payment
was a failed payment.

Fixes #4768
Re-choose best available promotion when an order changes

Before this change, we could end up with multiple eligible promotions on
the same order. This happened because changing the order (for example,
by adding line items) rechecked the eligibility of promotions,
potentially making multiple ones eligible, but forgot to re-choose the
one best promotion.

This change adds a call to the logic that chooses the best promotion any
time the ```OrderUpdater``` recalculates adjustments.

I believe this fixes #4675
Adds test to ensure we do not apply multiple line item promotions
Spec and style improvements

Runs two specs twice with different ordering to ensure the order in
which promos are applied doesn't matter. Also cleans up some style
issues and removes an unnecessary stub.