make Spec SEED more random

When the go-test executes multiple packages, they run concurrently with each other.
If the seed is purely time based, then they could generate the same values across different instances,
which is not good when a shared external resource is involved in testing with t.Random values.

seed.go

@@ -2,6 +2,8 @@ package testcase
 
 import (
 	"fmt"
+	"github.com/adamluzsi/testcase/random"
+	"math/rand"
 	"os"
 	"strconv"
 	"testing"
@@ -13,7 +15,9 @@ import (
 func makeSeed() (int64, error) {
 	rawSeed, injectedRandomSeedIsSet := os.LookupEnv(EnvKeySeed)
 	if !injectedRandomSeedIsSet {
-		return time.Now().UnixNano(), nil
+		salt := rand.New(random.CryptoSeed{}).Int63()
+		base := time.Now().UnixNano()
+		return base + salt, nil
 	}
 	seed, err := strconv.ParseInt(rawSeed, 10, 64)
 	if err != nil {