gdb: make set/show inferior-tty work with $_gdb_setting_str

Like the previous two commits, this commit fixes set/show inferior-tty
to work with $_gdb_setting_str.

Instead of using a scratch variable which is then pushed into the
current inferior from a set callback, move to the API that allows for
getters and setters, and store the value directly within the current
inferior.

Update an existing test to check the inferior-tty setting.

Reviewed-By: Tom Tromey <tom@tromey.com>