systemc: Switch to using predefined messages for channels.

Create and use predefined messages for channels which match the ones
Accellera uses.

Change-Id: I179214838bbd83604e50225926cdc6b5b1b16923
Reviewed-on: https://gem5-review.googlesource.com/c/13330
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Add some common test include files.

These are "common" in the sense that they're not in a particular test
directory, but I think they're only used by one test.

Change-Id: I4ffd209d04ed0e5253085810913827b87412b302
Reviewed-on: https://gem5-review.googlesource.com/11272
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>