select the AR adapter through `bin/test`.

actionmailer/bin/test

 #!/usr/bin/env ruby
 COMPONENT_ROOT = File.expand_path("../../", __FILE__)
 require File.expand_path("../tools/test", COMPONENT_ROOT)
+exit Minitest.run(ARGV)

actionpack/bin/test

 #!/usr/bin/env ruby
 COMPONENT_ROOT = File.expand_path("../../", __FILE__)
 require File.expand_path("../tools/test", COMPONENT_ROOT)
+exit Minitest.run(ARGV)

actionview/bin/test

 #!/usr/bin/env ruby
 COMPONENT_ROOT = File.expand_path("../../", __FILE__)
 require File.expand_path("../tools/test", COMPONENT_ROOT)
+exit Minitest.run(ARGV)

activemodel/bin/test

 #!/usr/bin/env ruby
 COMPONENT_ROOT = File.expand_path("../../", __FILE__)
 require File.expand_path("../tools/test", COMPONENT_ROOT)
+exit Minitest.run(ARGV)

activerecord/bin/test

 #!/usr/bin/env ruby
 COMPONENT_ROOT = File.expand_path("../../", __FILE__)
 require File.expand_path("../tools/test", COMPONENT_ROOT)
+module Minitest
+  def self.plugin_active_record_options(opts, options)
+    opts.separator ""
+    opts.separator "Active Record options:"
+    opts.on("-a", "--adapter [ADAPTER]",
+            "Run tests using a specific adapter (sqlite3, sqlite3_mem, mysql, mysql2, postgresql)") do |adapter|
+      ENV["ARCONN"] = adapter.strip
+    end
+
+    opts
+  end
+end
+
+Minitest.extensions.unshift 'active_record'
+
+exit Minitest.run(ARGV)

activesupport/bin/test

 #!/usr/bin/env ruby
 COMPONENT_ROOT = File.expand_path("../../", __FILE__)
 require File.expand_path("../tools/test", COMPONENT_ROOT)
+exit Minitest.run(ARGV)

tools/test.rb

@@ -8,5 +8,3 @@ def self.root
     @root ||= Pathname.new(COMPONENT_ROOT)
   end
 end
-
-exit Minitest.run(ARGV)