math/big: fix TestShiftOverlap for test -count arguments > 1
math/big: fix TestShiftOverlap for test -count arguments > 1

Don't overwrite incoming test data.

The change uses copy instead of assigning statement to avoid this.

Change-Id: Ib907101822d811de5c45145cb9d7961907e212c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/250137
Run-TryBot: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,golang/go,CAFxX/go,golang/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go