switch to minitest from test/unit

not sure if this is the right change, but tests seem to run

test/minimal_test_helper.rb

@@ -1,5 +1,4 @@
 ENV["RAILS_ENV"] ||= "test"
-require 'test/unit'
 require 'mocha/setup'
 
 $:.unshift File.dirname(File.dirname(__FILE__))

test/models/pie_chart_data_test.rb

@@ -2,7 +2,7 @@ require File.expand_path(File.dirname(__FILE__) + '/../minimal_test_helper')
 require 'app/models/stats/pie_chart_data'
 require 'active_support/core_ext/string'
 
-class Stats::PieChartDataTest < Test::Unit::TestCase
+class Stats::PieChartDataTest < Minitest::Test
 
   def setup
     xx = { :stats => { :other_actions_label => '(other)' } }

test/models/rich_message_extractor_test.rb

@@ -1,9 +1,8 @@
 require 'date'
-require 'test/unit'
 require 'active_support/core_ext/object/blank'
 require_relative '../../app/services/rich_message_extractor.rb'
 
-class RichMessageExtractorTest < Test::Unit::TestCase
+class RichMessageExtractorTest < Minitest::Test
 
   def test_message_with_all_options
     message = "ohai@some-context~this-project>131012<131014#tag1#tag2*"

test/models/staleness_test.rb

@@ -3,7 +3,7 @@ require_relative '../../lib/staleness'
 require_relative '../../lib/user_time'
 require 'timecop'
 
-class StalenessTest < Test::Unit::TestCase
+class StalenessTest < Minitest::Test
   FakePrefs = Struct.new(:time_zone)
   FakeUser = Struct.new(:time) do
     def prefs

test/models/tag_cloud_test.rb

@@ -1,7 +1,7 @@
 require File.expand_path(File.dirname(__FILE__) + '/../minimal_test_helper')
 require 'app/models/stats/tag_cloud'
 
-class TagCloudTest < Test::Unit::TestCase
+class TagCloudTest < Minitest::Test
 
   FakeTag = Struct.new(:name, :count)
 

test/models/time_to_complete_test.rb

@@ -5,7 +5,7 @@ require 'app/models/stats/time_to_complete'
 
 FakeTask = Struct.new(:created_at, :completed_at)
 
-class TimeToCompleteTest < Test::Unit::TestCase
+class TimeToCompleteTest < Minitest::Test
 
   def now
     @now ||= Time.utc(2013, 1, 2, 3, 4, 5)

test/models/tracks_cli/tracks_api_test.rb

@@ -4,7 +4,7 @@ require './doc/tracks_cli/tracks_api'
 
 module TracksCli
 
-  class TracksApiTest < Test::Unit::TestCase
+  class TracksApiTest < Minitest::Test
 
     def test_https_detection
       uri = URI.parse("https://tracks.example.com")

test/models/tracks_cli/tracks_xml_builder_test.rb

@@ -4,7 +4,7 @@ require 'active_support/time_with_zone'
 
 module TracksCli
 
-  class TracksXmlBuilderTest < Test::Unit::TestCase
+  class TracksXmlBuilderTest < Minitest::Test
     def test_all
       todo = {
         description: "test action",