boards/qemu_x86: Enable fast scheduler options

This target is already using the rbtree as part of CONFIG_USERSPACE,
so it incurs no code size overhead (actually it's a little smaller)
when using the scalable scheduler and waitq implementations.

The change also gets us test coverage of those choices on a default CI
target.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>