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>