Additional feature specs for new VSA features

What does this MR do?

Adds some additional feature specs for VSA

• Extracts some useful functions to helpers
• Adds a test for the Duration chart with a hidden stage - when the path navigation feature flag is enabled
• Wraps some additional tests a feature flag disabled context, to help with removal in !60449 (merged)

Covers scenarios that were previously only tested with the value_stream_analytics_path_navigation disabled

Related to #323982 (closed)

Screenshots (strongly suggested)

No visual changes

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because no user facing changes.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team