Extract frontend fixture creation from jest & karma

What does this MR do?

This MR extracts frontend test fixture creation from test jobs jest & karma into dedicated jobs.

It creates 2 new jobs in a new fixtures stage: frontend-fixtures and frontend-fixtures-foss. These jobs will create the required fixtures for subsequent frontend test jobs jest, karma, jest-foss and karma-foss respectively. The fixtures will be passed as artifacts across jobs.

The new fixture stage happens between prepare and test. jest, karma, jest-foss and karma-foss now depends on the jobs in fixture stage using DAG. There is no change to existing DAG of other jobs in test stage, so they will continue to start as soon as their dependencies in prepare completes.

It also adds new DAGs to jobs in test that did not have any DAG set up to avoid waiting for fixtures stage to complete.

What do we get out of this?

Immediate benefits

• reduced duplication of fixture creation, where previously each job creates its own fixtures
• reduced time to completion for test stage. Since jest is the last job to complete in the test stage, we compare the duration from the start of the pipeline (start of setup-test-env) to the end of jest job.

	pipeline	setup-test-env start time	jest job end time	jest duration	time to test completion
baseline	https://gitlab.com/gitlab-org/gitlab/pipelines/117477945	Thu Feb 13 05:56:54	Thu Feb 13 06:44:52	42m 02s	~48 minutes
with separate fixtures jobs	https://gitlab.com/gitlab-org/gitlab/pipelines/117487307	Thu Feb 13 06:41:26	Thu Feb 13 07:21:36	15m 11s	~40 minutes

Future benefit

• Now that jest and karma jobs purely execute tests, they can now be parallelised to split up the tests, without the overhead of creating fixtures (jest: !25104 (merged))

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

Part of #34322 (closed)

/cc @gl-quality/eng-prod