Freeze time in spec/lib/gitlab/analytics/unique_visits_spec.rb

What does this MR do?

Freeze the time in spec/lib/gitlab/analytics/unique_visits_spec.rb

We need to freeze to a reference time because visits are grouped by the week number in the year. Without freezing the time, the test may behave inconsistently depending on which day of the week test is run.

Test reason for test failure is explained in #223799 (comment 366084466)

Screenshots

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

Closes #223799 (closed)