--fail-on-focus command line switch to fail the suites when focus rockets found

This adds the command line option to override the `allowFocusMode` (default: `true`) in `buster-test` (see: https://github.com/busterjs/buster-test/pull/26)

This completes essential work for https://github.com/busterjs/buster/issues/327 - remaining work is to display a pretty error message in addition to failing the suite.