Adds missing API tests

[JayShortway]

This adds a few API tests that were missing. KobanKat didn't have them at all at first, and later I added only the most important ones. We were discussing removing these API tests since we have the .api files. However this was a good exercise for me to go over the API one more time. 😄

The PR is quite big, but the commits are self contained. Here's a description for each commit to help reviewing. Happy to split the PR if needed!

1. c96d666

This one adds API tests for enums. A when over an expect enum cannot be made exhaustive in a common source set, because any platform might have more cases that the common source set doesn't know about. That's why the API tests for these enums are in both iosMain and androidMain, essentially duplicated. There's probably something smart we can do with placing them in a single folder and adding that folder to both iosMain and androidMain. However, since we were talking about maybe removing API tests altogether (since we have the .api files), I think this is fine for now.

2. 4ffe9f3

Some PurchasesConfiguration options were not API-tested yet. These are added here.

3. 9957ea4

SubscriptionOption did not have the presentedOfferingContext field. This commit adds that and removes the deprecated presentedOfferingId. Deprecated instead in eed2ae8.

4. abde9ef

This one adds Paywall API tests, as these didn't exist at all yet.

5. addfde4

This one adds the updated .api file. (Caused by 9957ea4 and eed2ae8).