Fix flaky test failure in charts_spec

What does this MR do?

The timezone based tests in ci/charts_spec will fail when executed close to midnight, as witnessed in https://gitlab.com/gitlab-org/gitlab/-/jobs/1019524415

This change addresses the stability of the test by specifying an explicit time for an object that is being queried in a sensitive time window

This is a follow-up fix for just the tests originally added via !52971 (merged)

Screenshots (strongly suggested)

None, just a test change

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