Modify innodb.mysqld_core_dump_without_buffer_pool_dynamic test to move assertion to correct location

The test is unstable and sometimes fails with this assertion:

2019-11-20T16:45:43.232618Z 0 [ERROR] [MY-013183] [InnoDB] Assertion failure: srv0start.cc:3502:buf_flush_active_lru_managers() == srv_buf_pool_instances || buf_flush_active_lru_managers() == 0 thread 140199887362240

This test needs core pattern with pid so it is skipped on many platforms. Seen this failing on ubuntu Xenial.