* Migrate Tensorflow and TFLite in the CI up to 1.15.2

The latest stable version of Tensorflow and Tensorflow lite
in the 1.x series is 1.15.2. The tflite frontend is receiving
support for versions of tflite > 1.14 but there is no consistent
testing.

There are 2 failures already in the source base with tf 1.15
and I'm concerned this will just get exacerbated over time
if we don't have CI picking this up and I view this as a stepping
stone towards stepping CI to TF2.x.

The test failures that I have commented will get issues raised
for them as issues to be fixed.

* Comment out run of qnn_mobilenet_v3_net

This is another test that fails with TFlite 1.15.2

* Skip the qnn_mobilenet_v3 test in the pytest fashion.

* Switch docker versions to support Tensorflow 2.1.0

* Fix up pytest imports and usage.

* Skip these tests currently for Tensorflow 2.1.0