remove --str flag from db subcommands

The --str flag assumes that all the information will be passed as CLI
arguments, but that's no longer the case since we started using the
MYSQL_PWD env variable.

And we might want to start using the PDO extension in the future,
instead of the `mysql` binary.

Also introduce run_mysql_query() utility.
create tests database if it doesn't exist
update db man page
Merge pull request #374 from wp-cli/tests-db

`wp core init-tests` should create the database