Bugfixes in SHOW CREATE TABLE and SHOW TABLES

[adamziel]

Fixes a series of crashes encountered when

• SHOW TABLES now returns a flat list, not an array of objects – I'm not sure here. It makes $tables = $wpdb->get_results("SHOW TABLES LIKE '{$prefix}%'", ARRAY_N); work as expected here, but is the right thing to return, or is this more about correct handling of the fetch mode, like ARRAY_N?
• SHOW tables isn't all uppercase
• There's no table tho SHOW CREATE: SHOW CREATE TABLE _no_such_table;
• The table identifier passed to SHOW CREATE is quoted:

'SHOW CREATETABLE `_tmp_table`;'

This PR makes the Sandbox site plugin work in Playground

cc @bgrgicak @brandonpayton @wojtekn

[adamziel]

This solves a real problem so I'll go ahead and merge, but let's revisit in case there's more to SHOW TABLES and returning a flat list. Also, GitHub says all checks are still in progress when I can see they passed: