* Add a new test case for conv operator (#329)

This new test case is based on issue #329 to increase
coverage on optimized kernel's data precision for conv operator.
In this test, input, output and filter are all 8 bits and filter
tensor is of dimension 8x3x3x3 with different scales per output channel.

TESTED= local test with x86 and HiFi4.

* Move large test data variable into its own folder
and use one flat top namespace instead of nested namespace.

BUG=195779890

Add support for 8x16 kernels in TFLM.

Support 16-bit activations and 8-bit weights for a subset of the TFLM kernels.

BUG=http://b/184839019

Confirmed that the following command passes:
```
./tensorflow/lite/micro/tools/ci_build/test_all_new.sh GITHUB_PRESUBMIT
```