add OnlyOneTest

lib/test/cdo/test_only_one.rb

@@ -0,0 +1,20 @@
+require_relative '../test_helper'
+require 'cdo/only_one'
+
+class OnlyOneTest < Minitest::Test
+  def only_one?
+    !!only_one_running?(__FILE__)
+  end
+
+  def test_only_one
+    r, w = IO.pipe
+    pid = fork do
+      w.puts only_one? # true
+      Process.wait(fork {w.puts only_one?}) # false
+    end
+    Process.wait pid
+    w.puts only_one? # true
+    w.close
+    assert_equal %w[true false true], r.readlines(chomp: true)
+  end
+end