Make scalars plugin tests mostly TF 2.x–compatible (#2522)
Make scalars plugin tests mostly TF 2.x–compatible (#2522)

Summary:
These tests were disabled for two reasons: they used `tf.placeholder`,
and they used `tf.contrib.summary` for DB mode tests. This patch revises
the implementation of the non–DB mode tests to avoid using placeholders,
and enables those tests in all versions of TensorFlow. The DB mode tests
still cannot be run in TF 2.x because `create_db_writer` is not exported
under any public APIs.

Test Plan:
Tests pass in current `tf-nightly` and `tf-nightly-2.0-preview`.

wchargin-branch: scalars-plugin-tests-tf2x

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