Sliced tests doesn't reuse cached contexts when testing with Spock

Spock adds an annotation to a specification that makes each spec appear to be different:

```
@org.spockframework.runtime.model.SpecMetadata(filename=FilmController2Spec.groovy, line=18)
@org.spockframework.runtime.model.SpecMetadata(filename=FilmControllerSpec.groovy, line=18)
```

 Similar to what we did with Kotlin (#7101), we need to ignore Spock's annotations in `ContextCustomizerKey`.