private void runSharedInitializer(SpockExecutionContext context, SpecInfo spec) {
    if (spec == null) return;
    invoke(context, this, createMethodInfoForDoRunSharedInitializer(context, spec), spec);
  }
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