systemc: Merge stderr and stdout when running tests.

systemc: Merge stderr and stdout when running tests.

The golden output for tests doesn't distinguish between stderr and
stdout, and by only comparing against stdout we have to throw away
errors which would be good to verify we get right. Also the tests
sometimes send output to stderr for no apparent reason, requiring
manually patching the tests.

This change adds filters for two messages which used to go to stderr
in gem5 but now show up in the diffs, one that just says the simulation
is starting, and the other for warns of unimplemented functionality.
The second warning should be turned on at some point so we make sure
everything the tests touch works and they don't just work by
coincidence, but for now it introduces a lot of noise among otherwise
passing tests.

Change-Id: I3b14f7807af561a79d6e0ca87aff1ab6051be596
Reviewed-on: https://gem5-review.googlesource.com/c/12600
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>