pcu: Refactor GPRS infrastructure to keep state and simplify tests

Before this patch, each test had to somehow keep state for all the
transactions needed. Now, most of the state is moved to generic GprsMS,
UlTbf and DlTbf structures, and APIs to maintain its state, as well as
function helpers to submit or receive messages from it. For now
specially the Tx side was improved, some of the Rx parts are left for
later and are still using the old APIs.

This will allow for more complex scenarios and more complex tests
running several MS.

All the tests were updated to use the new APIs, reworked when needed and
even totally rewritten in some cases since they were doing
inconsistent/wrong stuff from the point of view of what the scenarios
or code paths they were expected to test. There's no test regressions.

Change-Id: Ib3fee37580f0ea0530a659dec83656799bf57288
2 files changed