tests: switch to Minitest 5

Update the test suite to use the new Minitest 5 framework API. Remove
the last references to Test::Unit and standardize on "Minitest"
everywhere.

mono_logger.gemspec

@@ -21,5 +21,5 @@ Gem::Specification.new do |spec|
 
   spec.add_development_dependency "bundler", "~> 1.3"
   spec.add_development_dependency "rake"
-  spec.add_development_dependency "minitest", "~> 4.0"
+  spec.add_development_dependency "minitest", "~> 5.0"
 end

test/mri_logger_test.rb

@@ -9,14 +9,14 @@
 require 'coveralls'
 Coveralls.wear!
 
-require 'test/unit'
+require 'minitest/autorun'
 require 'mono_logger'
 require 'tempfile'
 
 Logger = MonoLogger
 
 
-class TestLoggerSeverity < Test::Unit::TestCase
+class TestLoggerSeverity < Minitest::Test
   def test_enum
     logger_levels = Logger.constants
     levels = ["WARN", "UNKNOWN", "INFO", "FATAL", "DEBUG", "ERROR"]
@@ -29,7 +29,7 @@ def test_enum
 end
 
 
-class TestLogger < Test::Unit::TestCase
+class TestLogger < Minitest::Test
   include Logger::Severity
 
   def setup
@@ -270,7 +270,7 @@ def test_lshift
   end
 end
 
-class TestLogDevice < Test::Unit::TestCase
+class TestLogDevice < Minitest::Test
   class LogExcnRaiser
     def write(*arg)
       raise 'disk is full'
@@ -344,9 +344,7 @@ class << (stderr = '')
     end
     $stderr, stderr = stderr, $stderr
     begin
-      assert_nothing_raised do
-        logdev.write('hello')
-      end
+      logdev.write('hello')
     ensure
       logdev.close
       $stderr, stderr = stderr, $stderr
@@ -491,7 +489,7 @@ def test_shifting_age
 end
 
 
-class TestLoggerApplication < Test::Unit::TestCase
+class TestLoggerApplication < Minitest::Test
   def setup
     @app = Logger::Application.new('appname')
     @tempfile = Tempfile.new("logger")