Verilator concurrency issue about chisel-testers HOT 5 OPEN

@shunshou I'm pretty surprised that removing that line fixes things. All that says is don't run firrtl as part of the chisel.Driver.execute. I'll look over your changes more carefully right now

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@chick see my latest PR. It's not that line. I was wrong :\ changed too many things at once...

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@shunshou The problem is definitely caused by chisel-testers using the same directory for different tests, the number of files created when using the verilator backend seems to increase the likelihood of the error. I have created a branch in dsptools that fixes the problem with a work-around that overrides the directory. This is fix-simpletb-dir-stomping. I am not suggesting this is the final fix, but it is a short term workaround. The issues as I see them are:

• Two tests of the same dut get assigned the same dir by chisel-testers
• destructive stomping is more likely with verilator, but could happen with any backend
• it's test_run_dir/dutname
• One fix might be to use timestamp instead of hashcode
• this fixes the cross stomping
• but leads to a new directory for each test every time it is run
• chisel-testers could some how automate the clearing of the directories after each test
• would need an easy override of this, because debugging often requires looking at these dirs
• another fix might be to default to serial execution of tests

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@chick i don't think you ever pushed anything new to that branch. It'd prob be really convenient, if, at least as a temporary patch, this gets merged in with master...

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Sorry, not sure how that happened. It's pushed now

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