Add tf broadcast-to testing

This patch adds BoradcastTo Op to the tf tests.
Did not add tflite testing because the tf.lite.TFLiteConverter converts
tf.broadcast-to to tfl.mul by 1.

Signed-off-by: Tai Ly <tai.ly@arm.com>
Change-Id: Icd372e619c318121c19eca87d5716bcd5fbbbb23