crypto/rand: use io.ReadFull in test

On Solaris versions before Solaris 11, the kernel will not
return more than 1040 on a single read from /dev/urandom.

R=golang-dev, agl, bradfitz, rsc, iant, dchest
CC=golang-dev
https://golang.org/cl/6113046

src/pkg/crypto/rand/rand_test.go

@@ -7,6 +7,7 @@ package rand
 import (
 	"bytes"
 	"compress/flate"
+	"io"
 	"testing"
 )
 
@@ -16,9 +17,9 @@ func TestRead(t *testing.T) {
 		n = 1e5
 	}
 	b := make([]byte, n)
-	n, err := Read(b)
+	n, err := io.ReadFull(Reader, b)
 	if n != len(b) || err != nil {
-		t.Fatalf("Read(buf) = %d, %s", n, err)
+		t.Fatalf("ReadFull(buf) = %d, %s", n, err)
 	}
 
 	var z bytes.Buffer