gomock/controller: use skip additional frame

[stephen]

Description
In 8321731, the callerInfo call
gets nested within an additional function call, but the frame skip
isn't updated. When used with mockgen, this causes the location frame
to unhelpfully point to the generated mock instead of the callsite in
the user's test.

Before:

... at .../mocks/mock_ecriface/mock_ECRAPI.go:612 because: there are no expected calls of...

After:

... at .../example_test.go:29 because: there are no expected calls of...

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes tests

Reviewer Notes

• The code flow looks good.
• Tests added.

Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

- Fixes blame location for unexpected calls to mock

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[codyoss]

Thanks for the contribution. I think we should just document the magic number a little.

[stephen]

@codyoss thanks for looking. i added a commit documenting both usages of callerInfo and explaining the skip number.

[codyoss]

Thank you for your contribution!