crypto: centralize external test module fetches

crypto: centralize external test module fetches

This has the important advantage of using the system GOMODCACHE when it
exists, avoiding the download on every "go test".

While at it, also consistently use testenv.Command.

Change-Id: Ic999ffa281f6da73fe601b0feba29e60982cce3d
Reviewed-on: https://go-review.googlesource.com/c/go/+/628755
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
TryBot-Bypass: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>