test_chars.py races with itself when run in parallel

 TestStringQueries.test_varchar[exec_option: {'batch_size': 0, 'num_nodes': 0, 'disable_codegen_rows_threshold': 0, 'disable_codegen': True, 'abort_on_error': 1, 'exec_single_node_rows_threshold': 0} | table_format: text/none] 
[gw3] linux2 -- Python 2.7.12 /home/tarmstrong/Impala/incubator-impala/bin/../infra/python/env/bin/python
tests/query_test/test_chars.py:82: in test_varchar
    self.run_test_case('QueryTest/chars', vector)
tests/common/impala_test_suite.py:424: in run_test_case
    self.__verify_results_and_errors(vector, test_section, result, use_db)
tests/common/impala_test_suite.py:297: in __verify_results_and_errors
    replace_filenames_with_placeholder)
tests/common/test_result_verifier.py:404: in verify_raw_results
    VERIFIER_MAP[verifier](expected, actual)
tests/common/test_result_verifier.py:231: in verify_query_result_is_equal
    assert expected_results == actual_results
E   assert Comparing QueryTestResults (expected vs actual):
E     'hello' == 'hello'
E     None != 'hello'
E     Number of rows returned (expected vs actual): 1 != 2