skip concurrent read and write tests in short mode

Signed-off-by: Cenk Alti <cenkalti@gmail.com>

concurrent_test.go

@@ -48,6 +48,9 @@ TestConcurrentReadAndWrite verifies:
     following reading transactions (with txid >= previous writing txid).
 */
 func TestConcurrentReadAndWrite(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping test in short mode.")
+	}
 	bucket := []byte("data")
 	keys := []string{"key0", "key1", "key2", "key3", "key4", "key5", "key6", "key7", "key8", "key9"}