Support multiple tests from the command line.

It seems that support for multiple test files specified on the command line was lost in the [switch from optimist to bossy](https://github.com/hapijs/lab/commit/f5e42d8bb3123f3ade95bac521b81e266d9df7a8). Without this feature, tools like [gulp-lab](https://github.com/otodockal/gulp-lab) are broken. This change adds a test case for this scenario and restore the ability to specify multiple test scripts from the command line.