[SPARK-17611][YARN][TEST] Make shuffle service test really test auth.

Currently, the code is just swallowing exceptions, and not really checking
whether the auth information was being recorded properly. Fix both problems,
and also avoid tests inadvertently affecting other tests by modifying the
shared config variable (by making it not shared).

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #15161 from vanzin/SPARK-17611.