message: Fix reference computation in client_library_test_base for multi device case
  The constructor taking an execution option to specify multiple devices
  didn't created a reference client resoulting in a SEGV and then later it
  tried to execute the reference computation on multiple devices on the
  interpreter what isn't supported.
diff: @@ -61,6 +61,11 @@ ClientLibraryTestBase::ClientLibraryTestBase(
  : client_(GetOrCreateLocalClientOrDie(client_options)),
  execution_options_(CreateDefaultExecutionOptions()) {
  CHECK_EQ(platform, client_options.platform());
  +
  + LocalClientOptions ref_options;
  + ref_options.set_platform(GetReferencePlatform());
  + ref_client_ = GetOrCreateLocalClientOrDie(ref_options);
  +
  // Disabling constant_folding so that tests (usually written using Constants)
  // will exercise the intended code paths, instead of being constant folded.
  //
  @@ -152,6 +157,7 @@ ClientLibraryTestBase::ExecuteAndTransferReference(
  *execution_options.mutable_shape_with_output_layout() =
  *shape_with_output_layout;
  }
  + execution_options.clear_device_handles();
  return ref_client_->ExecuteAndTransfer(computation, arguments,
  &execution_options);
  }
  