scons,systemc: disable systemc tests scons by default

scons,systemc: disable systemc tests scons by default

Not running the systemc test SConscript reduces the scons startup time
(before any file is compiled) from about 10s to 4s on my machine.

The performance investigation was done at:
https://gem5.atlassian.net/browse/GEM5-256

As before, the systemc tests are still automatically built when
they are run with:

src/systemc/tests/verify.py --update-json build/ARM -j `nproc` \
  --filter-file src/systemc/tests/working.filt

Change-Id: I33b7a53c0a7d70386ab17d7bb4886c84a97a2eb3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25385
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>