allowFocusMode option

Part of https://github.com/busterjs/buster/issues/327

When testRunner has `opt.allowFocusMode` and it is set to fail - the test suite will not be run, but will fail instead.

Work remaining:

- [x] ~~actual CLI option in `buster-test-cli`~~ https://github.com/busterjs/buster-test-cli/pull/14
- [x] Error message display when `runSuite()` fails (need advice!)