systemc: Teach verify.py to filter messages which start within a line.

systemc: Teach verify.py to filter messages which start within a line.

It's possible for a test to generate a warning or error in the middle
of a line of output. The previous filter generator function would
create a filter which would only detect those messages which started
at the beginning of a new line.

Change-Id: I40372dc33049df84f3111e4d63a6619db97dcaa3
Reviewed-on: https://gem5-review.googlesource.com/c/12597
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>