E2E: Add waiter in bulk_import_group validation spec

Description of the test

This started as attempt to debug/fix #331704 (closed). Turned out that waiting logic doesn't actually fix the problem but I think this kind of matcher can still be useful in the future so leaving it.

• Adds waiter for import validation which since some of the recent changes uncovered flakiness, incorrect behaviour
• Introduces a more convenient way to do so via eventually_ rspec matcher. In case such matcher fails, the produced error message would look something like this:

Failure/Error: expect { imported_group.labels }.to eventually_include(*source_group.labels).within(duration: 5)

Wait condition not met after 5 seconds:
expected to eventually include {
  "name": "source-group-fc1b15d6",
  "description": "This is a test label",
  "description_html": "This is a test label",
  "color": "#0033CC",
  "text_color": "#FFFFFF",
  "subscribed": false
}, last attempt was []

Check-list

• 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.