IMPALA-800 workaround. Mark test_libs_with_same_filename as serial.

This test will drop functions in a binary used by the other UDF
tests. That triggers IMPALA-800.

Change-Id: I8e6f1ad5b4a7ece2d891559751142f0c12e07c3c
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1556
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Tested-by: Lenni Kuff <lskuff@cloudera.com>