Add e2e tests for externally managed members

[CaptainIRS]

How to categorize this PR?

/area testing
/kind test

What this PR does / why we need it:

Adds e2e tests for the externally managed members feature introduced in #1214.

Which issue(s) this PR fixes:
To aid with testing for the upcoming member removal feature #1307.

Checklist:

• Add tests that cover your changes (if applicable)
  • E2E tests

Special notes for your reviewer:

If you have an existing kind cluster, it has to be brought down (make kind-down) and brought up again with this PR branch (make kind-up) for this to work.

Release note:

NONE

[gardener-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[CaptainIRS]

Placeholder comment to remove this once gardener/etcd-backup-restore#939 is part of the latest release.

[gardener-prow]

@CaptainIRS: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-etcd-druid-api-unit	9454cda	link	false	/test pull-etcd-druid-api-unit

Full PR test history. Your PR dashboard. Command help for this repository.
Please help us cut down on flakes by linking this test failure to an open flake report or filing a new flake report if you can't find an existing one. Also see our testing guideline for how to avoid and hunt flakes.

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