test.py: Return test output only if subprocess succeeded

The current code will try to print the output of a
subprocess.Popen().communicate() call even if that
call raised an exception and that output is None.

Let's fix this problem by only printing the output
if it's not None.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@scylladb.com>

test.py

@@ -149,9 +149,10 @@ if __name__ == "__main__":
                     print_status('TIMED OUT\n')
                 else:
                     print_status('  with error code {code}\n'.format(code=proc.returncode))
-                print('=== stdout START ===')
-                print(str(out, encoding='UTF-8'))
-                print('=== stdout END ===')
+                if out is not None:
+                    print('=== stdout START ===')
+                    print(str(out, encoding='UTF-8'))
+                    print('=== stdout END ===')
                 all_ok = False
             else:
                 print_status('%s PASSED %s' % (prefix, path))