If there are any failures, return a non-zero exit status so that
invoking scripts can detect that failures happen. This eliminates
the need to use regression report reference outputs, and should
make it easier to merge PRs that add or impact tests.
By adding ivtest to the iverilog source tree, it is easier to keep
the regression test synchronized with the source that is being tested.
This should be especially helpful for PRs that add a new feature, and
have a matching ivtest PR with the regression test for that feature.
Stephen Williams