Add ProcessLogger to TesterDriver for less test noise (#2574)

Captures default output in TesterDriver with a ProcessLogger using the
Logger from FIRRTL to suppress output from Verilator when running tests.
This will change behavior for any users of the TesterDriver because the
standard out of Verilator compilation and execution is suppressed by default.

Users can create their own ProcessLogger to restore the previous
behavior, eg.

import scala.sys.process.ProcessLogger
val echoLogger = ProcessLogger(Console.out.println, Console.err.println)