Change selectedSuite to use index

What does this MR do?

This is for the test report tab on the pipeline details page.

selectedSuite in the test reports store was setting the whole suite object. This worked, but we are moving towards not having the whole suite object loaded at first. If we were to keep this, then when clicking on the suite, the report wouldn't show any test cases because they hadn't loaded yet.

Instead, we can set the index of the selectedSuite so that when we load the whole suite we can still use the index to get the testCases from the suite.

This is a non user-facing refactor, so no changelog is necessary.

Related to #218725 (closed)

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