tests/iwyu.py: print diff if descrepancy is found

tests/iwyu.py

@@ -4,18 +4,23 @@ from __future__ import print_function
 
 import json
 import subprocess
+import shlex
 
 with open("compile_commands.json", "r") as f:
     tunits = json.load(f)
 
 result = 0
 for tu in tunits:
-    _,rest = tu["command"].split(" ",1)
+    _,cmd = tu["command"].split(" ",1)
+    cmd = "%s %s"%("iwyu", cmd)
+    cmd = shlex.split(cmd)
     # iwyu does not distinguish between different outcomes of its check
     # so instead, we grep its stderr output
     # see http://code.google.com/p/include-what-you-use/issues/detail?id=157
-    ret = subprocess.call("%s %s 2>&1 | grep \"has correct #\""%("iwyu", rest), shell=True)
-    if ret != 0:
+    ret = subprocess.Popen(cmd, stderr=subprocess.PIPE)
+    _,ret = ret.communicate()
+    if "has correct #" not in ret:
         result += 1
+        print(ret)
 
 exit(result)