Detect when a compilation error will prevent a test from running

[sarahec]

Recommended enhancement to #8 -- split the compilation and running stages into two parts (allowing the compiler's stderr and stdout to appear normally), and check the returned result. If compilation failed with an error, show a warning that we're running the previous version of the test.

This allows the watch command to run without erroring out and keeps running the un-afflicted tests.

(Kindly assign to me. Thanks)

[sarahec]

Turns out there's a much easier way, at least for icarus verilog: delete the a.out file at before compiling. I'm submitting a patch momentarily.