Several MYSQL_SYSVAR_STR parameters that employ both a validate
function callback fail to copy the string for saving the
validated value. The affected variables include the following:

innodb_ft_aux_table
innodb_ft_server_stopword_table
innodb_ft_user_stopword_table
innodb_buffer_pool_filename

The test case is an enhanced version of
mysql/mysql-server@0b0c30641fd66336e87394ac28587e40864f8af9
and the code changes are inspired by their fixes.

We are also importing and adjusting the test innodb_fts.stopword
to get coverage for the variable innodb_ft_user_stopword_table.

buf_dump(), buf_load(): Protect srv_buf_dump_filename with
LOCK_global_system_variables.

fts_load_user_stopword(): Minor cleanup

fts_load_stopword(): Remove the parameter global_stopword_table.

innobase_fts_load_stopword(): Protect innodb_server_stopword_table
against concurrent SET GLOBAL.