openpgp/clearsign: just use rand.Reader in tests

Instead of a convoluted fake rand, it is _basically_ just as fast, and
fixes errors that pop up due to bad entropy.

Fixes golang/go#70682

Change-Id: Ib0f605398d1092b516b03135f602c644be2a060f
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/633655
Reviewed-by: Tatiana Bradley <tatianabradley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

openpgp/clearsign/clearsign_test.go

@@ -123,24 +123,12 @@ func TestSigning(t *testing.T) {
 	}
 }
 
-// We use this to make test keys, so that they aren't all the same.
-type quickRand byte
-
-func (qr *quickRand) Read(p []byte) (int, error) {
-	for i := range p {
-		p[i] = byte(*qr)
-	}
-	*qr++
-	return len(p), nil
-}
-
 func TestMultiSign(t *testing.T) {
 	if testing.Short() {
 		t.Skip("skipping long test in -short mode")
 	}
 
-	zero := quickRand(0)
-	config := packet.Config{Rand: &zero}
+	var config packet.Config
 
 	for nKeys := 0; nKeys < 4; nKeys++ {
 	nextTest: