consensus/ethash: increase seal timeout for tests (#22162)

It seems that the 2 second timeout is not enough for Travis CI:

   --- FAIL: TestTestMode (2.00s)
       ethash_test.go:53: sealing result timeout

consensus/ethash/ethash_test.go

@@ -49,7 +49,7 @@ func TestTestMode(t *testing.T) {
 		if err := ethash.VerifySeal(nil, header); err != nil {
 			t.Fatalf("unexpected verification error: %v", err)
 		}
-	case <-time.NewTimer(2 * time.Second).C:
+	case <-time.NewTimer(4 * time.Second).C:
 		t.Error("sealing result timeout")
 	}
 }