UnexpectedErrors may reference exceptions that can't be dumped

UnexpectedError exceptions wrap the original exception, and the original
exception may contain a reference to something that can't be marshal
dumped which will cause the process to die.

actionpack/test/abstract_unit.rb

@@ -484,6 +484,9 @@ def shutdown
           method   = job[1]
           reporter = job[2]
           result = Minitest.run_one_method klass, method
+          if result.error?
+            translate_exceptions result
+          end
           queue.record reporter, result
         end
       }
@@ -491,6 +494,20 @@ def shutdown
     @size.times { @queue << nil }
     pool.each { |pid| Process.waitpid pid }
   end
+
+  private
+  def translate_exceptions(result)
+    result.failures.map! { |e|
+      begin
+        Marshal.dump e
+        e
+      rescue TypeError
+        ex = Exception.new e.message
+        ex.set_backtrace e.backtrace
+        Minitest::UnexpectedError.new ex
+      end
+    }
+  end
 end
 
 if ActiveSupport::Testing::Isolation.forking_env? && PROCESS_COUNT > 0