Disable admin preview for extensions test apps

The new admin is far from complete and existing
extensions will fail if we test them with the
new default admin that we generate apps with.

It can be enabled by setting ADMIN_PREVIEW=true
in your test env.

core/lib/spree/testing_support/common_rake.rb

@@ -31,6 +31,7 @@ def initialize
           'solidus:install',
           Dir.pwd, # use the current dir as Rails.root
           "--auto-accept",
+          "--admin-preview=#{ENV.fetch('ADMIN_PREVIEW', 'false')}",
           "--authentication=none",
           "--payment-method=none",
           "--migrate=false",