VReplication: CODEOWNERS and unit test housekeeping

[mattlord]

Description

This PR makes two minor changes:

1. Adds tabletmanager/vdiff to GitHub CODEOWNERS (I noticed that it was not there from a recent PR)
2. Makes two minor unit test adjustments to deal with golangci-lint failures now on main:

❯ $(go env GOPATH)/bin/golangci-lint run go/... --timeout 10m
go/vt/vtctl/workflow/traffic_switcher_test.go:969:20: Error return value of `ts.cancelMigration` is not checked (errcheck)
      ts.cancelMigration(ctx, sm)
                        ^
go/vt/vtctl/workflow/traffic_switcher_test.go:1030:20: Error return value of `ts.cancelMigration` is not checked (errcheck)
      ts.cancelMigration(ctx, sm)
                        ^

This occurred because of two PRs that got merged concurrently today (new unit tests and changes to cancelMigration).

Related Issue(s)

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

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
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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 67.76%. Comparing base (fd1186c) to head (f8ed4d9).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17646   +/-   ##
=======================================
  Coverage   67.75%   67.76%           
=======================================
  Files        1586     1586           
  Lines      255726   255763   +37     
=======================================
+ Hits       173261   173307   +46     
+ Misses      82465    82456    -9     

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