Flakes: Address flakiness in TestZkConnClosedOnDisconnect

[mattlord]

Description

This is a quick folllow-up to #17136 as the new unit test quickly proved to be flaky in the CI (example).

It also failed virtually every time for me locally. It was a timing issue where we restarted ZooKeeper and immediately tried to connect, but it was not yet ready to accept new connections. We address that by waiting until we are able to connect.

We should backport this test fix to v19, as we did the original PR.

Note: I confirmed that the new test, as it is in this PR, still fails on a branch without the fixes in #17136:

--- FAIL: TestZkConnClosedOnDisconnect (6.20s)
    zk_conn_test.go:59:
        	Error Trace:	/Users/matt/git/vitess/go/vt/topo/zk2topo/zk_conn_test.go:59
        	Error:      	An error is expected but got nil.
        	Test:       	TestZkConnClosedOnDisconnect
FAIL
FAIL	vitess.io/vitess/go/vt/topo/zk2topo	8.435s
FAIL

Related Issue(s)

• Follow-up to Close zookeeper topo connection on disconnect #17136

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.34%. Comparing base (8af57c0) to head (29dd0c0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17194      +/-   ##
==========================================
+ Coverage   67.32%   67.34%   +0.02%     
==========================================
  Files        1569     1569              
  Lines      252548   252548              
==========================================
+ Hits       170018   170070      +52     
+ Misses      82530    82478      -52     

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