crypto/internal/cryptotest: add SkipTestAllocations

crypto/internal/cryptotest: add SkipTestAllocations

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[    It has been [ 0 ] days since Filippo broke a TestAllocations.    ]
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Concentrate all the skips in one place, so we don't have to re-discover
always the same ones via trial and error.

This might over-skip fixable allocations, but all these targets are not
fast anyway, so they are not worth going back for.

Removed the sysrand TestAllocations because it causes an import loop
with cryptotest and it's covered by TestAllocations in crypto/rand.

Change-Id: Icd40e97f9128e037f567147f8c9dafa758a47fac
Reviewed-on: https://go-review.googlesource.com/c/go/+/626438
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>