Prevent the use of unknown test modifiers.
import test = from 'tape';
test.onlu(t => {});
test.seril(t => {});
test.only.onlu(t => {});
test.beforeeach(t => {});
test.unknown(t => {});
import test = from 'tape';
test.only(t => {});
test.skip(t => {});
test.skip.only(t => {});