Slicing a given model up to the given first N operations

Also regenerated all CTS, VTS and unit tests, as a test harness function
is renamed.

Test: test.py and sliced mobilenet_quantized up to first 20 ops
Test: unit tests and VTS on Sailfish
Bug: 67781676
Change-Id: I68dcdbda17f8bedc98b1da573a8f884da0d5722d