test.py: check test xml output
test.py: check test xml output

Check that XML output of a test is valid and warn otherwise.

The following tests currently produce a warning:
boost/multishard_mutation_query_test

Message-Id: <f3f5923b2b939981994b5562dbf5469b16080493@scylladb.com>

avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla