generated test harness should support the --help option

I don't know how to run the test harness (beyond just running the generated binary).  I tried several times to learn how to run it by passing it the `--help` option, but ha ha, that option was not supported by the generated test harness.

It looks like it would be pretty easy to extend the test harness code (src/libextra/test.rs) with `--help` and -h` options to get such info.
