[test] fix a few jndi_mysql tests
[test] fix a few jndi_mysql tests

pool.active_connection?  returns the actual connection or nil, not false.

kares/activerecord-jdbc-adapter,kares/activerecord-jdbc-adapter,jruby/activerecord-jdbc-adapter,bruceadams/activerecord-jdbc-adapter,bruceadams/activerecord-jdbc-adapter,jruby/activerecord-jdbc-adapter,jruby/activerecord-jdbc-adapter,bruceadams/activerecord-jdbc-adapter,kares/activerecord-jdbc-adapter