Refactored and extended test double code; added PDU tests

This is some work-in-progress on MRP test cases I wanted to put 
up for comment before I go too much further along this path. My
goal was to make minimal changes to the code under test, but to
get enough instrumentation to isolate the packet parsing code. 

I moved the test double code that stands in for mrpd's functions
into its own files, mrp_doubles.c|h. I extended several of them to
track the calls made by the application during tests. I also
started a test group for PDU parse tests and added test double
infrastructure to observe the parse results fairly directly.
I determined that msrp_event() is the only function that is called during the parsing process in msrp_rcv_msg() to produce actions. Since msrp_rcv_msg() takes no parameters and returns nothing beyond a binary sucess/fail value, instrumenting it can only be done via direct interposition of its calls via a test double.

Most of the other test doubles are for functions defined in another file, but this one was defined in the *same* file, so we can't select the double via the linker. This `#define` just renames the original when testing so we can wrap or bypass it to limit the amount of code being executed during tests.  I like to make unit tests at the finest level of granularity possible, as it gives a clear indication (assuming the code itself is granular enough) of where failure is occurring and gives me much more confidence that it fulfills the assumptions I place upon it when writing/examining code that relies upon it.

Once you are at the level of an entire MSRP system observing another MSRP system, you're moving beyond unit testing to a level of integration/behavior testing. That is definitely something we ought to do, but unit tests as such need to be able to directly supply inputs and observe outputs of a piece of code *in isolation*.  I don't want to have to reason backward from state machine transitions, or worse, from transmitted PDUs, to determine the correctness of PDU *parsing*. :)