test: Add missing unit tests in vtgate/cli

[Devanshusharma2005]

Description

This PR adds the missing unit tests for vtgate CLI (go/cmd/vtgate/cli)

Test command:

cd go/cmd/vtgate/cli
go test ./... -race -count=1 -coverprofile=coverage.out -covermode=atomic

Result:

Related Issue(s)

Fixes part of #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

Deployment Notes

[vitess-bot]

Review Checklist

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• Ensure that the Pull Request has a descriptive title.
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[mattlord]

We should avoid adding test specific things to the production code if at all possible.

[Devanshusharma2005]

Rolled back the cli changes. Will remember this for future test additions. ^^

[Devanshusharma2005]

@mattlord ^^

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.40%. Comparing base (70c7a72) to head (a6092bb).
⚠️ Report is 37 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (70c7a72) and HEAD (a6092bb). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (70c7a72)	HEAD (a6092bb)
	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #19439       +/-   ##
===========================================
- Coverage   69.67%   57.40%   -12.27%     
===========================================
  Files        1614       17     -1597     
  Lines      216793      439   -216354     
===========================================
- Hits       151044      252   -150792     
+ Misses      65749      187    -65562     

Flag	Coverage Δ
partial	57.40% <ø> (?)

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[mattlord]

LGTM. Please see my note about using ErrorContains in a consistent way in these new tests. Please address that while we wait for the second reviewer. Thanks, @Devanshusharma2005 !