Added unit tests for vt/grpcclient package

[VaibhavMalik4187]

Description

Bumps the coverage of the go/vt/grpcclient package to 70%

Related Issue(s)

Partially Addresses: #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

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

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.67%. Comparing base (9d861f8) to head (02e5a50).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15072      +/-   ##
==========================================
+ Coverage   65.49%   65.67%   +0.18%     
==========================================
  Files        1562     1562              
  Lines      193940   193940              
==========================================
+ Hits       127016   127378     +362     
+ Misses      66924    66562     -362     

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[VaibhavMalik4187]

@GuptaManan100 could you please take a look at these tests? Thanks!

[GuptaManan100]

Rest looks good to me!

[GuptaManan100]

LGTM now. There is a conflict though, you might need to merge main