Add spec for Process.spawn searching the executable in the passed env PATH

* From #2419

Author: eregon

spec/ruby/core/process/spawn_spec.rb

@@ -212,6 +212,24 @@
     end.should output_to_fd("nil\n")
   end
 
+  it "uses the passed env['PATH'] to search the executable" do
+    dir = tmp("spawn_path_dir")
+    mkdir_p dir
+    begin
+      exe = 'process-spawn-executable-in-path'
+      path = "#{dir}/#{exe}"
+      File.write(path, "#!/bin/sh\necho $1")
+      File.chmod(0755, path)
+
+      env = { "PATH" => "#{dir}#{File::PATH_SEPARATOR}#{ENV['PATH']}" }
+      Process.wait Process.spawn(env, exe, 'OK', out: @name)
+      $?.should.success?
+      File.read(@name).should == "OK\n"
+    ensure
+      rm_r dir
+    end
+  end
+
   it "calls #to_hash to convert the environment" do
     o = mock("to_hash")
     o.should_receive(:to_hash).and_return({"FOO" => "BAR"})