[#307] v2/tombstone/test: Do not allocate memory if !empty

Move all memory allocation and field settings
in `Generate...(empty bool)` functions behind
`if !empty` check.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

v2/tombstone/test/generate.go

@@ -11,9 +11,8 @@ func GenerateTombstone(empty bool) *tombstone.Tombstone {
 	if !empty {
 		m.SetExpirationEpoch(89)
 		m.SetSplitID([]byte{3, 2, 1})
+		m.SetMembers(refstest.GenerateObjectIDs(false))
 	}
 
-	m.SetMembers(refstest.GenerateObjectIDs(empty))
-
 	return m
 }