data: simplify dispatching provider tests (#3858)
data: simplify dispatching provider tests (#3858)

Summary:
Per comment of @nfelt on #3847, we can still test for delegate equality:
<https://github.com/tensorflow/tensorboard/pull/3847#discussion_r457698571>

wchargin-branch: data-dispatch-test-bare

tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard