crypto/internal/fips140test: make entropy file pair names match

crypto/internal/fips140test: make entropy file pair names match

Change-Id: I6a6a69642d00e3994277d9b5631d1d7f18f3f356
Reviewed-on: https://go-review.googlesource.com/c/go/+/710055
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>