api: add test-harness feature

api: add test-harness feature

If enabled, the Schema type implements Eq and PartialEq for
testing the api macro better.

Note that these implementations don't make all too much
since since they also compare `dyn Fn` types which do not
implement Eq. Since they're also `&'static` they can't
really be runtime closures, so this should be fine, we know
they'll always point to some regular function.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>