Serializing dependency in test script(s)?

```
fftgen issue: test-regress.sh & maybe other tests as well
- maybe cannot run two instances at the same time :(

test-regress.sh BUG
- i.e. this fails:
    test-regress.sh --perl   -sim vcs       >& tmp-regress-pl-vcs.log &
    test-regress.sh --python -sim verilator >& tmp-regress-py-ver.log &
    test-regress.sh --perl   -sim verilator >& tmp-regress-pl-ver.log &
```
