Print stdin from Scala tests to a file

This allows me to run some chisel code that uses the Scala test
harness without Scala running.  This is nice because it allows me to
run these tests as part of my automatic test harness, without having
to deal with all the SBT stuff that I don't understand.
Add "--dumpTestInput" as an argument

This will cause the stdin that's read by the tester to get dumped to a
file during every run.  The file will be named "Design.stdin", where
"Design" is the name of the top-level design that's being run (it's
the same as the rest of the generated files, like "Design.cpp" or
"Design.vcd").

I'm currently using this functionality in order to attempt to get
completely determintsic tests so I can run both the C++ backend and my
new LLVM backend and expect to get the same result.

It could be useful for running tests on embedded systems that can't
run Scala (like the Zedboard).
Merge pull request #151 from palmer-dabbelt/print_stdin

Allow access to the Scala tester's test vectors