Load test badge counter asynchronously

What does this MR do?

Step 1: to make Junit Test Report faster as explained on #37725 (closed)

This MR loads the test badge counter asynchronously by making an AJAX request after the page has been loaded by the browser.

This is the first step of small performance improvement we are doing to make our new feature JUnit Test Report more resilient.

Screenshots

This MR will modify the data showing on our Testing Badge as you can see here:

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