tests: Don't treat new stats as a cause for failures

tests: Don't treat new stats as a cause for failures

We currently fail the stat diff stage of tests if there are new
stats. This is usually undesirable since this would require any change
that adds a stat to also update the regressions.

Change-Id: Ieadebac6fd17534e1b49b6b9a1d56f037a423325
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/3962
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>