go/vt/mysqlctl: fix dup open/close backup stats

[maxenglander]

Description

Found a silly bug (in code I wrote) where the Destination:Close and Source:Close stats are being reported multiple times. Removing the duplications and adding a test.

Related Issue(s)

Fixes #12946

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

[vitess-bot]

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[deepthi]

Why do you need to export this? I see no usages outside the package.

[maxenglander]

Huh can't remember why I did this.. will revert