E2E: Bulk group import api test

Description of the test

Splits bulk group import testing in to UI and API counterparts:

• Validate import can be done via UI
• Validate import can be done via API
• Further on, API counterpart will be used for validation of nested objects correct import since initial import is faster and more reliable. UI counterpart is only used for import page validation and smoke testing that import indeed happened.
• Minor boyscoutism, log attribute to debug level to be consistent with rest of the qa logging

Check-list

• Confirm the test has a testcase: tag linking to an existing test case in the test case project.
• Note if the test is intended to run in specific scenarios. If a scenario is new, add a link to the MR that adds the new scenario.
• Follow the end-to-end tests style guide and best practices.
• Use the appropriate RSpec metadata tag(s).
• Ensure that a created resource is removed after test execution. A Group resource can be shared between multiple tests. Do not remove it unless it has a unique path. Note that we have a cleanup job that periodically removes groups under gitlab-qa-sandbox-group.
• Ensure that no transient bugs are hidden accidentally due to the usage of waits and reloads.
• Verify the tags to ensure it runs on the desired test environments.
• If this MR has a dependency on another MR, such as a GitLab QA MR, specify the order in which the MRs should be merged.
• (If applicable) Create a follow-up issue to document the special setup necessary to run the test: ISSUE_LINK
• If the test requires an admin's personal access token, ensure that the test passes on your local with and without the GITLAB_QA_ADMIN_ACCESS_TOKEN provided.

Implementation: !66353 (merged)
Closes: #336918 (closed)