crypto/internal/fips140only: test fips140=only mode

crypto/internal/fips140only: test fips140=only mode

Fixes #70514
Updates #70878

Change-Id: I6a6a46561d872c8f7e9ea333ff208064b0bd44c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/728506
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Michael Knyszek <mknyszek@google.com>