Resolve "jest 4/4 failing (timezone)"

What does this MR do?

On Nov 1, 2020 Alaska moved from DST to AKST, this shifts the UTC timezone offset from -8 hours to -9 hours. The fixture data for spec/frontend/deploy_freeze/components/timezone_dropdown_spec.js was correctly updated, but the test was still expecting the hardcoded -8 hours for the UTC offset.

• Ensure we use the fixture data to compare the dropdown result instead of a hardcoded value
• Moves formatUtcOffset to a shared utility
• Renamed formatUtcOffset to secondsToHours

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

Related to #275842 (closed)