[subset] Fix deadlock waiting for process output in subset test runner.

test/subset/run-tests.py

@@ -19,9 +19,9 @@ from subset_test_suite import SubsetTestSuite
 def cmd(command):
 	p = subprocess.Popen (
 		command, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-	p.wait ()
-	print (p.stderr.read (), end="") # file=sys.stderr
-	return p.stdout.read (), p.returncode
+	(stdoutdata, stderrdata) = p.communicate()
+	print (stderrdata, end="") # file=sys.stderr
+	return stdoutdata, p.returncode
 
 def read_binary(file_path):
 	with open(file_path, 'rb') as f: