Avoid loading fast_spec_helper if spec_helper is loaded

What does this MR do?

Avoid loading fast_spec_helper if spec_helper is loaded

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Related to #249992 (closed)