Fix CI error with minitest 5.14

Follow up to 2bf1c17c20, which still left
this one failure. Minitest 5.14 uses error instead of exception, so
exception had been changed to just be the same UnexpectedError instance
and losing us the actual error message.

This also switches to initializing a new UnexpectedError instance
because previously, for reasons I can't quite figure out, reusing the
same instance would still give us a DRbConnError.

activesupport/lib/active_support/testing/parallelization.rb

@@ -101,8 +101,14 @@ module ActiveSupport
               begin
                 queue.record(reporter, result)
               rescue DRb::DRbConnError
-                result.failures.each do |failure|
-                  failure.send(:initialize, DRb::DRbRemoteError.new(failure.exception))
+                result.failures.map! do |failure|
+                  if failure.respond_to?(:error)
+                    # minitest >5.14.0
+                    error = DRb::DRbRemoteError.new(failure.error)
+                  else
+                    error = DRb::DRbRemoteError.new(failure.exception)
+                  end
+                  Minitest::UnexpectedError.new(error)
                 end
                 queue.record(reporter, result)
               end