systemc: Filter a pydot warning message out when checking test output.

systemc: Filter a pydot warning message out when checking test output.

This warning can show up if pydot isn't set up properly and doesn't have
anything to do with the success of the test.

Change-Id: Iddcea5aa27196bc5cf747bf5a295b6c9a91b3d2c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32174
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>