[#64] object/test: reuse checksum and signature generators

These were missing from #53.

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

signature/test/generate.go

@@ -0,0 +1,15 @@
+package test
+
+import (
+	"github.com/nspcc-dev/neofs-sdk-go/signature"
+)
+
+// Signature returns random pkg.Signature.
+func Signature() *signature.Signature {
+	x := signature.New()
+
+	x.SetKey([]byte("key"))
+	x.SetSign([]byte("sign"))
+
+	return x
+}