rabbitmq_peer_discovery_etcd: Wait for etcd start in system_SUITE (backport #12985)

[mergify]

Why

It was possible that testcases were executed before the etcd daemon was ready, leading to test failures.

How

There was already a santy check to verify that the etcd daemon was working correctly, but it was itself a testcase.

This patch moves this code to the etcd start code to wait for it to be ready.

This replaces the previous workaround of waiting for 2 seconds.

While here, log anything printed to stdout/stderr by etcd after it exited.

Fixes #12981.

---

This is an automatic backport of pull request #12985 done by Mergify.

[mergify]

Cherry-pick of d54b2bf has failed:

On branch mergify/bp/v4.0.x/pr-12985
Your branch is up to date with 'origin/v4.0.x'.

You are currently cherry-picking commit d54b2bff3.
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  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
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	both modified:   deps/rabbitmq_peer_discovery_etcd/test/system_SUITE.erl

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