void runInitializer(SpockExecutionContext context) {
    getSpecificationContext(context).setCurrentIteration(context.getCurrentIteration());
    runInitializer(context, context.getSpec());
  }
Executes a single Spec. Notifies its supervisor about overall execution
progress and every invocation of Spec code.
Supervisor also determines the error strategy.

@author Peter Niederwieser
spock-core/src/main/java/org/spockframework/runtime/PlatformSpecRunner.java