Improve consistency of tests.

Merged: https://github.com/ruby/ruby/pull/3323

test/fiber/scheduler.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'fiber'
+require 'socket'
 
 begin
   require 'io/nonblock'

test/fiber/test_enumerator.rb

@@ -1,19 +1,19 @@
 # frozen_string_literal: true
 require 'test/unit'
-require 'socket'
 require_relative 'scheduler'
 
 class TestFiberEnumerator < Test::Unit::TestCase
   MESSAGE = "Hello World"
 
   def test_read_characters
-    skip unless defined?(UNIXSocket)
+    skip "UNIXSocket is not defined!" unless defined?(UNIXSocket)
 
     i, o = UNIXSocket.pair
+
     unless i.nonblock? && o.nonblock?
       i.close
       o.close
-      skip
+      skip "I/O is not non-blocking!"
     end
 
     message = String.new

test/fiber/test_io.rb

@@ -6,10 +6,15 @@ class TestFiberIO < Test::Unit::TestCase
   MESSAGE = "Hello World"
 
   def test_read
-    skip unless defined?(UNIXSocket)
+    skip "UNIXSocket is not defined!" unless defined?(UNIXSocket)
 
     i, o = UNIXSocket.pair
-    skip unless i.nonblock? && o.nonblock?
+
+    unless i.nonblock? && o.nonblock?
+      i.close
+      o.close
+      skip "I/O is not non-blocking!"
+    end
 
     message = nil
 
@@ -31,5 +36,7 @@ class TestFiberIO < Test::Unit::TestCase
     thread.join
 
     assert_equal MESSAGE, message
+    assert_predicate(i, :closed?)
+    assert_predicate(o, :closed?)
   end
 end