test: Add missing tests for traffic_switcher.go

[beingnoble03]

Description

Adds some unit tests to cover traffic_switcher.go

Coverage diff:

• Coverage on main

ok  	vitess.io/vitess/go/vt/vtctl/workflow	2.733s	coverage: 62.6% of statements

• Coverage on test-traffic-switcher

ok  	vitess.io/vitess/go/vt/vtctl/workflow	2.862s	coverage: 64.6% of statements

Related Issue(s)

• Tracking Issue: vtctldclient/vtctld: Enhance test coverage and code refactor  #16499

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

• 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

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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

All modified and coverable lines are covered by tests ✅

Project coverage is 67.52%. Comparing base (747a61c) to head (7208a65).
Report is 21 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17334      +/-   ##
==========================================
+ Coverage   67.48%   67.52%   +0.04%     
==========================================
  Files        1577     1577              
  Lines      253424   253424              
==========================================
+ Hits       171033   171135     +102     
+ Misses      82391    82289     -102     

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[rohit-nayak-ps]

Just to confirm, we should check IsPrimaryServing before changeShardRouting