In order to run these tests you need a file named 'sandbox_config.json' in the current directory.
The file is formatted thusly:

{
  "public_key": "xxx",
  "merchant_id": "xxx",
  "private_key": "xxx"
}

You also need to have created elements in the control panel. These items are
verified by test preambles, but the description of how to create them is here.

Add-ons:
Create these by going to Recurring Billing > Add-ons / Discounts (left-side)
    * id=increase_30
      name="Increase 30"
      description="" (default)
      amount=30.00
      duration=subscription

Discounts:
Create these by going to Recurring Billing > Add-ons / Discounts (left-side)
    * id=discount_15
      name="Discount 15"
      description="" (default)
      amount=15.00
      duration=subscription

Subscription Plans:
Create these by going to Recurring Billing > Plans (left-side menu)
    * id=integration_trialless_plan
      name="Integration Trialless Plan"
      description="" (default)
      price=12.34
      include_trial_period=No (default)
      billing_cycle_every=1
      first_bill_date=immediately (default)
      billing_cycles=never_expires (default)
      No addons
      No discounts
    * id=integration_plan_with_add_ons_and_discounts
      name="Integration Plan with Add-ons and Discounts"
      description="" (default)
      price=1.00
      include_trial_period=Yes
          * trial_period=1 day
      billing_cycle_every=1
      first_bill_date=immediately (default)
      billing_cycles=never_expires (default)
      addons:
          * increase_30
      discounts:
          * discount_15

Merchant Accounts:
Create these by going to Settings > Processing (top-side menu). Scroll to the
bottom to the "Merchant Accounts" section.
    * id=sandbox_master_merchant_account
      default=Yes
      accept_paypal=Yes

NOTE: You must have a Marketplace account. The Marketplace features are only
enabled for US sandbox accounts. To have a Marketplace account, you have to have
selected "United States" as your country when signing up for your account. Then,
you can sign up for your sandbox with Marketplace features enabled.

Custom Fields:
Create these by going to Settings > Processing (top-side menu). Scroll down to
"Transactions" and you'll see "Custom Fields". Click on "Options" (tiny green
text on the right), then click "+ Add" (tiny green text in the upper right).
    * id=store_me
      display_name="Store Me"

Unfortunately, there is no way to verify the custom fields have been created via
the API, so TestHelper->verify_sandbox() cannot catch this error.

Transaction Descriptors:
You will need to have "dynamic descriptors" enabled for your account. There is
currently no way to determine if dynamic descriptors are enabled for an account
other than to ask Braintree support. To enable dynamic descriptors, you will
need to contact Braintree support.
