Allow overwriting REPORT_FILENAME + magic '-' for STDOUT

What does this MR do?

Allow overwriting REPORT_FILENAME

• treats - exceptionally! ( POSIX Utility Syntax Guidelines, §12.2.13 )
  sends the result to STDOUT instead of the file $APP_PATH/$REPORT_FILENAME

This if cat; else cat is not super-neat,
but the compound with $APP_PATH doesn't leave much choice while also trying to keep the change small...

And I´ve added tests so all 3 cases (normal-noname,named,stdout) are checked 🙂

IRL Usecase:

• when docker volumes are read-only the whole image "fails" currently
  allowing STDOUT as alternative provides a workaround

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template (also in CE)
  • Job definition example
  • CI Templates (to be removed)
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer