scripts: make 'scripts/test.py' support Python3

Python3 no longer allow implicitly conversion form bytes to string,
add explicit decode() to convert input bytes.

Tested with both Python2 and Python3.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Zifei Tong <zifeitong@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

scripts/test.py

-#!/usr/bin/env python2
+#!/usr/bin/env python
 import subprocess
 import argparse
 import glob
@@ -72,7 +72,7 @@ def run_test(test):
     out = ""
     line = ""
     while True:
-        ch = process.stdout.read(1)
+        ch = process.stdout.read(1).decode()
         if ch == '' and process.poll() != None:
             break
         out += ch