Defer test_reports request until tests tab is selected

What does this MR do?

This changes the behaviour of the test_report.json request on the pipeline details page. Instead of being invoked as part of the Javascript and Vue initialisation, the endpoint will now only be called if the Tests tab is specifically selected by the user.

This doesn't solve the root performance issue (being tracked in #37725 (closed)) but may help alleviate some stress. Also, I'm not sure if this is an approach we would consider, so this may not be merged but it might be worth keeping as a consideration.

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