Fix incorrect use of loop variable in parallel test

[renatolabs]

This commit fixes an occurrence of a loop variable being captured in a
parallel test. With very high probability, only the last test case
will actually be exercised. To work around this problem, we create a
local copy of the range variable before the parallel test, as advised
by the Go documentation at:

https://pkg.go.dev/testing#hdr-Subtests_and_Sub_benchmarks

Issue was found automatically using the loopvarcapture linter.

[vitess-bot]

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[mattlord]

Hi @renatolabs !

Can you please add a comment here as well just to be sure nobody comes along later and removes this "unnecessary" line. 🙂 Something like this:

// We create an explicit copy of the range variable for each parallel runner
// to be sure that they each run as expected. You can see more information on
// this here: https://pkg.go.dev/testing#hdr-Subtests_and_Sub_benchmarks
// The problem is covered by govet's rangeloop check:
//   https://github.com/golangci/govet/blob/master/rangeloop.go

Otherwise it LGTM. Thank you for contributing to Vitess! ❤️

[renatolabs]

Thanks for reviewing!

I included the comment as you suggested, except for the last two lines, as vet does not capture these types of bugs right now (golang/go#16520).

As a side note, the dynamic race detector that ships with Go (go test -race) is also not able to detect bugs of this type because of golang/go#35670.

[mattlord]

Thanks again! ❤️