Let `SurefireReportOnlyMojo` be a child of `AbstractSurefireReportMojo` to avoid execution of phase validate.

Because Surefire uses doclet annotations and there is no way to specify a phase `@execute phase="none"`,
`SurefireReportOnlyMojo` would inherit the execution of phase `test` from `SurefireReportMojo`.

Alternatives to `SurefireReportOnlyMojo` extending from `AbstractSurefireReportMojo` and duplicating setters etc.:
* Introduce another abstract class between `AbstractSurefireReportMojo` and `SurefireReportMojo` which
  defines the setters etc. and let both `SurefireReportMojo` and `SurefireReportOnlyMojo` extend this, only
  defining different goal-names and only `SurefireReportMojo` define `@execute`.
* Switch to Java annotations instead of doclet annotations.

Both approaches would have had a much bigger risk of introducing incompatibilities.