systemc: Remove references to internal constants from the tests.

systemc: Remove references to internal constants from the tests.

The tests were referring to constants defined in the Accellera systemc
implementation which identified various log message types. This change
replaces those (sometimes quite long) string constants with their
actual value. This doesn't make that interface any more fragile since
the constant value won't track between the Accellera version and this
one, but it does make its fragility more explicit by using the value
directly.

If in the future we decide to hide the magical nature of those
particular string constants, we could make them into our own constants
in the implementation. Regardless, the tests shouldn't refer to
internal details of the implementation.

Change-Id: I116a7407fcac49367649b250683117f3ccf4f9ed
Reviewed-on: https://gem5-review.googlesource.com/11181
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>