test: Include unit tests for switcher-dry-run

[beingnoble03]

Description

This PR adds unit tests for switcher_dry_run.go.

Coverage diff:

• Coverage on main

ok  	vitess.io/vitess/go/vt/vtctl/workflow	2.733s	coverage: 64.6% of statements

• Coverage on tests-switcher-dr

ok  	vitess.io/vitess/go/vt/vtctl/workflow	2.862s	coverage: 67.2% 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]

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• 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 67.61%. Comparing base (45192d2) to head (8c8edbf).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17403      +/-   ##
==========================================
+ Coverage   67.52%   67.61%   +0.08%     
==========================================
  Files        1581     1581              
  Lines      253948   253945       -3     
==========================================
+ Hits       171480   171696     +216     
+ Misses      82468    82249     -219     

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[mattlord]

Thanks, @beingnoble03 ! Just the one nit about the struct name.

[mattlord]

IMO we should rename this to newSwitcherDryRunEnv so that it's more aligned with what it is.