tests: add tests for vt/vtgate/engine/opcode

[Maniktherana]

Description

Bumps coverage to 100% for vt/vtgate/engine/opcode

Related Issue(s)

#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

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

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• 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.

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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: 435 lines in your changes are missing coverage. Please review.

Comparison is base (eddb39e) 47.29% compared to head (76ed127) 47.49%.
Report is 60 commits behind head on main.

Files	Patch %	Lines
...vt/vtgate/planbuilder/operators/sharded_routing.go	0.00%	42 Missing ⚠️
go/vt/vtgate/semantics/early_rewriter.go	86.41%	23 Missing and 10 partials ⚠️
go/vt/vtgate/planbuilder/operators/delete.go	0.00%	32 Missing ⚠️
.../vtgate/planbuilder/operators/delete_with_input.go	0.00%	31 Missing ⚠️
go/vt/vtgate/engine/delete_with_input.go	53.70%	23 Missing and 2 partials ⚠️
...tgate/planbuilder/operators/aggregation_pushing.go	0.00%	25 Missing ⚠️
...vt/vtgate/planbuilder/operators/queryprojection.go	0.00%	20 Missing ⚠️
.../vt/vtgate/planbuilder/operators/query_planning.go	0.00%	19 Missing ⚠️
go/vt/vtgate/evalengine/expr_tuple_bvar.go	62.50%	14 Missing and 4 partials ⚠️
go/vt/vtgate/planbuilder/operators/phases.go	0.00%	16 Missing ⚠️
... and 45 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15045      +/-   ##
==========================================
+ Coverage   47.29%   47.49%   +0.19%     
==========================================
  Files        1137     1149      +12     
  Lines      238684   239324     +640     
==========================================
+ Hits       112895   113661     +766     
+ Misses     117168   117076      -92     
+ Partials     8621     8587      -34     

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[GuptaManan100]

LGTM!