WIP: Fix e2e tests that fail when run in parallel

What does this MR do?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29654 allows e2e tests to be run in parallel.

The changes caused a few tests to start failing. For example, because of a race condition when creating the sandbox group. Those failures will be fixed in this MR so that the original MR can be merged without having to wait for all the fixes.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content