Backport 17.11: Generate separate project and group work items fixtures

What does this MR do and why?

Important

This MR is %17.11 backport of !189201 (merged)

Fix test fixtures

spec/frontend/fixtures/work_items.rb and ee/spec/frontend/fixtures/work_items.rb generate text fixtures to the same file graphql/work_items/namespace_work_item_types.query.graphql.json. This appears to cause a race condition on the CI pipelines which causes jest tests to non-deterministically use one output or another

References

• Fix #537479 (closed).
• gitlab-org/quality/engineering-productivity/master-broken-incidents#12365 (closed)
• gitlab-com/gl-infra/delivery#21006 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

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