Reduce complexity during import phase

In the current state of the negative testing framework the import phase
consists in many complex operation during import phase. This fix reduces
the complexity during that phase to improve the performance,
debugability and robustness.

In this fix we only determine the test scenarios in the import phase. The
payloads are created while test execution. This also improves the
flexibility since the test executor has full access to the test variables.

Change-Id: I8deed33d37c5bf07c26118d6d66ada105d259c89
Partially-implements: bp autogen-negative-tests
Merge "Reduce complexity during import phase"