Connectors: Gate default setting auto-registration on `is_active`.

Update `_wp_register_default_connector_settings()` to register a connector's
default API key setting only when the connector's `plugin.is_active` callback
returns `true`. 
Add tests covering the gate's branches: setting skipped when `is_active`
returns `false`, setting registered when it returns `true`.

Props jorgefilipecosta, gziolo, peterwilsoncc.
Fixes #65099.

git-svn-id: https://develop.svn.wordpress.org/trunk@62289 602fd350-edb4-49c9-b593-d223f7449a82