Fix ActiveRecord adapters not being loaded for testing

Gemfile

@@ -11,4 +11,12 @@ gemspec
 gem 'solidus_extension_dev_tools', github: 'solidusio-contrib/solidus_extension_dev_tools'
 gem 'sprockets', '~> 3'
 gem 'sprockets-rails'
-gem 'sqlite3'
+
+case ENV['DB']
+when 'postgresql'
+  gem 'pg'
+when 'mysql'
+  gem 'mysql2'
+else
+  gem 'sqlite3'
+end

spec/support/dummy_app/database.yml

@@ -1,4 +1,16 @@
+<% if ENV['DB'] == 'postgresql' %>
+test:
+  adapter: postgresql
+  database: circle_test
+  username: root
+<% elsif ENV['DB'] == 'mysql' %>
+test:
+  adapter: mysql2
+  database: circle_test
+  username: root
+<% else %>
 test:
   adapter: sqlite3
   database: ':memory:'
   timeout: 10000
+<% end %>