discovery: Fix tablets removed from healthcheck when topo server GetTablet call fails

[brendar]

Description

This PR fixes two bugs that can appear when using Zookeeper as the topo server

1. topo.Server GetTabletsIndividuallyByCell will return an empty slice on a partial result error from GetTabletMap, which causes TopologyWatcher to remove all tablets from the healthcheck.
2. After fixing the first bug, TopologyWatcher will still remove any tablets not present in the partial results list.

In order to test these fixes this PR also introduces a more flexible way to return fake errors from memorytopo.

Since the impact of this bug could be pretty disruptive, I think it should be backported to v19.

Related Issue(s)

Fixes #15632

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

Deployment Notes

n/a

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 75.43860% with 14 lines in your changes are missing coverage. Please review.

Project coverage is 68.13%. Comparing base (0e2f175) to head (1c40017).
Report is 12 commits behind head on main.

Files	Patch %	Lines
go/vt/topo/memorytopo/lock.go	40.00%	6 Missing ⚠️
go/vt/topo/memorytopo/file.go	70.00%	3 Missing ⚠️
go/vt/topo/memorytopo/watch.go	50.00%	2 Missing ⚠️
go/vt/topo/memorytopo/directory.go	50.00%	1 Missing ⚠️
go/vt/topo/memorytopo/election.go	50.00%	1 Missing ⚠️
go/vt/topo/tablet.go	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15633      +/-   ##
==========================================
- Coverage   68.13%   68.13%   -0.01%     
==========================================
  Files        1556     1556              
  Lines      194984   195032      +48     
==========================================
+ Hits       132849   132881      +32     
- Misses      62135    62151      +16     

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[deepthi]

I believe this is the real fix for a bug originally reported in #3987 !!
EDIT: Not exactly the same, but a similar bug. And a similar fix would work.

[deepthi]

This is great work. The testing improvements are particularly nice.
We'll get another review from @mattlord but LGTM
We don't need to backport beyond v19 because we only started using GetTablets in v19.

[mattlord]

Excellent work on this @brendar ! Thank you for the great contribution ❤️