Fixes [CF-684] and #1594.
`SystemInfo.appleSubscriptionsURL` was a short URL that didn't pass the `SystemInfo.isAppleSubscription(managementURL:)` test. This is fixed and covered by a test now.
### Other changes:
- Made `SystemInfo.isAppleSubscription(managementURL:)` and `SystemInfo.appleSubscriptionsURL` static since they don't need any properties in `SystemInfo`.
- Forced-unwrapped `SystemInfo.appleSubscriptionsURL` to remove unnecessary error handling. This is safe because the URL is provided statically at compile time. Per the docs "This initializer returns nil if the string doesn’t represent a valid URL. For example, an empty string or one containing characters that are illegal in a URL produces nil.". This is known at compile time, and covered by tests, so it could never return `nil`.
- Combined redundant tests in `ManageSubscriptionsHelper`. They were all doing exactly the same for both platforms.
[CF-684]: https://revenuecats.atlassian.net/browse/CF-684?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ
NachoSoto