…re (#2532)

See https://app.circleci.com/jobs/github/RevenueCat/purchases-ios/69881

Assertion failures in that method were leading to _crashes_ in
`Purchases.shared`, instead of a simple test failure that could be
retried:

![image](https://github.com/RevenueCat/purchases-ios/assets/685609/c86e17a8-a0f4-4b97-a4b8-ee1537d6378e)

For some reason, `XCTest` is continuing execution after an assertion
failure in an `async` method, despite having `self.continueAfterFailure
= false`.
This might even be a Swift mis-compilation. This PR is a workaround:
instead of relying on `XCTest` stopping execution of the test, we
instead throw an `Error` to force that at compile time.

Note that this still produces nice errors when an assertion fails inside
of `verifyEntitlementWentThrough`:
![Screenshot 2023-05-23 at 13 27
55](https://github.com/RevenueCat/purchases-ios/assets/685609/5a6aa599-235e-41c8-9d85-930e55ace1b1)

Fixes https://app.circleci.com/pipelines/github/RevenueCat/purchases-ios/11501/workflows/b978f26f-ce2d-4040-ac40-96e121c4407f/jobs/72997/steps

Same as #2532, this prevents test crashes when this function is called after `Purchases` has been reset.
Test crashes are inconvenient because it makes the entire test run fail, instead of allowing retries.

Fixes
https://app.circleci.com/pipelines/github/RevenueCat/purchases-ios/11501/workflows/b978f26f-ce2d-4040-ac40-96e121c4407f/jobs/72997/steps

Same as #2532, this prevents test crashes when this function is called
after `Purchases` has been reset.
Test crashes are inconvenient because it makes the entire test run fail,
instead of allowing retries.

…perty

This is a similar change to #2532.

I noticed yet another source of crashes in our integration tests.
This is what the result was:
```
failed - Message 'Finishing transaction' should not have been logged
expected to not find object in collection that satisfies predicate
```

That came from this code:
```swift
self.logger.verifyMessageWasNotLogged("Finishing transaction")

let info1 = try await Purchases.shared.customerInfo()
```

After that assertion failure, `XCTest` continued execusion, while it had already tear down `Purchases`, which lead to this crash:
```
Thread 0 Crashed:
0   libswiftCore.dylib            	       0x103b84e81 _assertionFailure(_:_:file:line:flags:) + 353
1   RevenueCat                    	       0x1332516d0 static Purchases.shared.getter + 352 (Purchases.swift:65)
2   BackendIntegrationTests       	       0x130f7048a OfflineStoreKit1IntegrationTests.testPurchaseWhileServerIsDownPostsReceiptWhenForegroundingApp() + 362 (OfflineStoreKitIntegrationTests.swift:289)
3   BackendIntegrationTests       	       0x130f75ee1 @objc closure #1 in OfflineStoreKit1IntegrationTests.testPurchaseWhileServerIsDownPostsReceiptWhenForegroundingApp() + 1
4   BackendIntegrationTests       	       0x130f77d91 partial apply for @objc closure #1 in OfflineStoreKit1IntegrationTests.testPurchaseWhileServerIsDownPostsReceiptWhenForegroundingApp() + 1
```

To prevent this, instead of using `Purchases.shared` this replaces that with a throwing `self.purchases`.
Just like we don't use force-unwraps in tests and instead use `XCTUwnrap`, this now will fail the test instead of crashing it.

…perty (#2867)

This is a similar change to #2532.

I noticed yet another source of crashes in our integration tests.
This is what [the
result](https://app.circleci.com/pipelines/github/RevenueCat/purchases-ios/13247/workflows/52fd2014-7a19-442f-a572-64116393abec/jobs/94728)
was:
```
failed - Message 'Finishing transaction' should not have been logged
expected to not find object in collection that satisfies predicate
```

That came from this code:
```swift
self.logger.verifyMessageWasNotLogged("Finishing transaction")

let info1 = try await Purchases.shared.customerInfo()
```

After that assertion failure, `XCTest` continued execution, while it had
already tear down `Purchases`, which lead to this crash:
```
Thread 0 Crashed:
0   libswiftCore.dylib            	       0x103b84e81 _assertionFailure(_:_:file:line:flags:) + 353
1   RevenueCat                    	       0x1332516d0 static Purchases.shared.getter + 352 (Purchases.swift:65)
2   BackendIntegrationTests       	       0x130f7048a OfflineStoreKit1IntegrationTests.testPurchaseWhileServerIsDownPostsReceiptWhenForegroundingApp() + 362 (OfflineStoreKitIntegrationTests.swift:289)
3   BackendIntegrationTests       	       0x130f75ee1 @objc closure #1 in OfflineStoreKit1IntegrationTests.testPurchaseWhileServerIsDownPostsReceiptWhenForegroundingApp() + 1
4   BackendIntegrationTests       	       0x130f77d91 partial apply for @objc closure #1 in OfflineStoreKit1IntegrationTests.testPurchaseWhileServerIsDownPostsReceiptWhenForegroundingApp() + 1
```

To prevent this, instead of using `Purchases.shared` this replaces that
with a throwing `self.purchases`.
Just like we don't use force-unwraps in tests and instead use
`XCTUwnrap`, this now will fail the test instead of crashing it.