PCU: revert changes to f_TC_egprs_pkt_chan_req(), fix tests

I noticed that TC_egprs_pkt_chan_req_reject_exhaustion has started
to fail since the refactoring change [1]. As it turned out, this
test case does not init a GprsMs instance, which is needed by a
function it depends on - f_TC_egprs_pkt_chan_req().

I don't really see the point of using the GprsMs abstraction for
such small test cases sending just a few messages, which do not
require us to keep any state. Let's just revert those changes,
and keep using the low level API.

[1] Ib3fee37580f0ea0530a659dec83656799bf57288

Change-Id: I4f32c138e7587e68d2d35d13d13c044893fec4b2
1 file changed