integration_test: Do not throw away stdout/stderr.
integration_test: Do not throw away stdout/stderr.

When nothing fails, output is ignored, but when something fails, you'd
want to look at the error messages.

BUG=None
TEST=time ./builder --vm ./run_tests

Change-Id: If6e73588b33714dea1e7f097a91c80f7c249be48
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3083207
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>
Commit-Queue: Junichi Uekawa <6bd240a6ce32463e272bb69c685a8e946b33ac05@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm