crypto/rand: skip TestAllocations if optimizations are off

crypto/rand: skip TestAllocations if optimizations are off

Without optimizations escape analysis can't do as much.

Updates #66779

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-noopt
Change-Id: I9ccd1b995c62427ceebd9ce5c98170dbf4a93e8b
Reviewed-on: https://go-review.googlesource.com/c/go/+/618275
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>