[Brightbox] Remove dependency on .fog in test

This isolates the tests from the values in `Fog.credentials` which is
based on the contents of `~/.fog`

Underlying problem is that the code (and many others) rely on the Fog
global making it too tightly coupled to the environment hence having to
stub.

Full issue: https://github.com/fog/fog/issues/1390

This will probably be a repeating pattern we need to work on to DRY up
so every test from every provider doesn't include this setup workaround.

providers/brightbox/test/fog/compute/brightbox_test.rb

@@ -3,8 +3,24 @@ require "fog/brightbox"
 
 class Fog::Compute::BrightboxTest < Minitest::Test
   def setup
-    @arguments = {}
-    @service = Fog::Compute::Brightbox.new(@arguments)
+    @arguments = {
+      :brightbox_auth_url => "http://localhost",
+      :brightbox_api_url => "http://localhost",
+      :brightbox_client_id => "",
+      :brightbox_secret => "",
+      :brightbox_username => "",
+      :brightbox_password => "",
+      :brightbox_account => ""
+    }
+
+    @credential_guard = Minitest::Mock.new
+    def @credential_guard.reject
+      {}
+    end
+
+    Fog.stub :credentials, @credential_guard do
+      @service = Fog::Compute::Brightbox.new(@arguments)
+    end
   end
 
   def test_respond_to_request