Update mini pipeline selectors to use data-testid

What does this MR do?

Fixes #300407 (relevant to the Vue implementation)

Update mini pipeline selectors to use data-testid

Currently the class mini-pipeline-graph-dropdown-toggle is used in CSS and tests, which makes refactoring difficult.

This change updates one of the widely used mini pipeline classes to use data-testid in feature tests.

Screenshots (strongly suggested)

N/A

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

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

Related to #299902 (closed)