Resolve "JUnit XML summary incorrectly shows 100% success rate"

What does this MR do?

Changes the behaviour of the rounding for the success percentage displayed in the Junit reports view. Percentages are now rounded down using Math.floor. While this could still producing rounding issues, we're not displaying a breakdown of percentages or summing in any way, so this is a fairly quick fix that will catch most scenarios.

In the future, if we do need to display a breakdown or sum, then we may need to reconsider how we round to whole numbers.

Screenshots

No visual changes.

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

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

Closes #37363 (closed)