Unlink Tempfiles after use in SpawnProcess

On Windows, the finalizer for Tempfile will raise an error if
Errno::ENOACCES is raised when removing the file. However, if this
happens when #unlink is called, the error is caught.

This change calls #unlink on the Tempfiles that are used to store the
output of spawned processes. This means the finalizer will not need to
unlink the file, so any Errno::ENOACCES errors during unlinking will be
caught.

The raised errors did not make any specs fail, but they did appear in
the rspec output.

lib/aruba/processes/spawn_process.rb

@@ -360,6 +360,7 @@ def read_temporary_output_file(file)
         file.rewind
         data = file.read
         file.close
+        file.unlink
 
         data.force_encoding("UTF-8")
       end