tests: add tests for go/slice

[ChaitanyaD48]

Description

Adds unit tests for the slice package. The tests cover the cases resulting in 100% code coverage.

Related Issue(s)

Improve Unit Test Coverage #14931

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

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

Attention: 309 lines in your changes are missing coverage. Please review.

Comparison is base (eddb39e) 47.29% compared to head (7670d1c) 47.39%.
Report is 46 commits behind head on main.

Files	Patch %	Lines
...vt/vtgate/planbuilder/operators/sharded_routing.go	0.00%	40 Missing ⚠️
go/vt/vtgate/semantics/early_rewriter.go	84.81%	25 Missing and 11 partials ⚠️
...tgate/planbuilder/operators/aggregation_pushing.go	0.00%	25 Missing ⚠️
...vt/vtgate/planbuilder/operators/queryprojection.go	0.00%	20 Missing ⚠️
go/vt/vttablet/tabletmanager/rpc_query.go	58.69%	14 Missing and 5 partials ⚠️
go/vt/vtgate/evalengine/expr_tuple_bvar.go	62.50%	14 Missing and 4 partials ⚠️
go/vt/vttablet/grpctmclient/client.go	0.00%	15 Missing ⚠️
go/vt/vtgate/evalengine/cached_size.go	0.00%	10 Missing ⚠️
...gate/planbuilder/operators/info_schema_planning.go	0.00%	10 Missing ⚠️
go/vt/vttablet/tabletserver/tabletserver.go	33.33%	7 Missing and 3 partials ⚠️
... and 50 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14989      +/-   ##
==========================================
+ Coverage   47.29%   47.39%   +0.09%     
==========================================
  Files        1137     1146       +9     
  Lines      238684   239197     +513     
==========================================
+ Hits       112895   113356     +461     
- Misses     117168   117257      +89     
+ Partials     8621     8584      -37     

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