Remove print statements from Python tests

Use exceptions instead of printing to stdout.
Part of an effort to convert Python tests to python 3 syntax.

Examples/test-suite/python/li_attribute_runme.py

@@ -8,12 +8,10 @@ if aa.a != 1:
     raise RuntimeError
 aa.a = 3
 if aa.a != 3:
-    print aa.a
-    raise RuntimeError
+    raise RuntimeError("aa.a: {}".format(aa.a))
 
 if aa.b != 2:
-    print aa.b
-    raise RuntimeError
+    raise RuntimeError("aa.b: {}".format(aa.b))
 aa.b = 5
 if aa.b != 5:
     raise RuntimeError