[release-branch.go1.15] 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.

For #45335

Change-Id: Ib907101822d811de5c45145cb9d7961907e212c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/250137


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
(cherry picked from commit 41bc0a17)
Reviewed-on: https://go-review.googlesource.com/c/go/+/319831


Run-TryBot: Carlos Amedee <carlos@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Carlos Amedee <carlos@golang.org>
Reviewed-by: Jonathan Albrecht <jonathan.albrecht@ibm.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>