sql: export `sql_current_time` && enable tests

`sql_current_time` is exported only in debug mode and used to check
builtin datetime sql functions behavior in specific time moments.

Extra changes:
 * table-14.1 test is deleted, as it does not test anything.
   It was testing ddl inside of a transaction. This case is checked
   by 14.2 by now.

Closes #2646