test_analytic_fns is flaky (small fractional differences in AVG)

The issue occurred here:
https://jenkins.impala.io/job/ubuntu-16.04-dockerised-tests/2065/testReport/query_test.test_queries/TestQueries/test_analytic_fns_protocol__hs2___exec_option____batch_size___0___num_nodes___0___disable_codegen_rows_threshold___0___disable_codegen___False___abort_on_error___1___exec_single_node_rows_threshold___0____table_format__parquet_none_/

The same test failed with all protocols.

Failing query:
https://github.com/apache/impala/blob/ebbe52b4bed944d3012e3679dc984827ce11d5a8/testdata/workloads/functional-query/queries/QueryTest/analytic-fns.test#L2

Errors are like (hs2):
2009,3,6,3.666666666666667,2009-03-01 20:12:00.475000,'0','8' != 2009,3,6,3.66666666667,2009-03-01 20:12:00.474999,'0','8'

I think that the problem is not the difference in the 4th double column, as that depends on the client used and we do not require complete match during comparison. So the problem is likely to be the timestamp in the 5th column.