MDEV-29853  Assertion `!strstr(table->name.m_name, "/FTS_") || purge_sys.must_wait_FTS()' failed in trx_t::commit

  - Failing debug assertion is to indicate whether the purge thread
is waiting when fts auxilary table is being dropped. But assertion
fails if the table name contains FTS_. So in fts_drop_table(), InnoDB
sets the auxilary table flag in transaction modified table list.