Make frontend fixtures parallel with knapsack

What does this MR do?

This MR enables the use of parallel jobs to generate frontend-fixtures, while also splitting the ee and non-ee fixtures in separate jobs.

Screenshots (strongly suggested)

This results in changes in the fixtures stage:

• rspec frontend_fixture
• rspec frontend_fixture as-if-foss
• rspec-ee frontend_fixture, now run using parallel: 2.

Before	After

Follow-up

I would like open a follow up to split up the slowest fixture ee/spec/frontend/fixtures/analytics.rb, so that parallel is used more efficiently.

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 #217916 (closed)