Closes: #8221 (More context on #8256)

Description

This PR restores the tests for InAppPurchaseAction.userIsEntitledToProduct, which were removed due to their flakiness, and attempt to resolve the issue for the time being by simulating the expected entitlements response we're waiting from StoreKit.

From testing, the core of the issue seems to be that we cannot fully reset the state of SKTestSession between different runs, despite calling both resetToDefaultState() and clearTransactions() on tearDown we can still see shared state between different tests. We can confirm this by removing the call to .clearTransactions(), which makes all entitlement tests pass or fail based on the order we run them.

Changes

• Restore skipped tests from UnitTests.xctestplan
• Adds a transactions property which acts as SKTestTransaction transactions holder
• Adds a simulateUserIsEntitledToProduct(with id: String) helper method which will return true or false depending on the contents of the transactions array. If the user already has transactions, we'll return false, otherwise we'll return true.

A further improvement could be to use VerificationResult<Transaction> type, so we can test and capture different type of errors as well depending on the case, which is why the method also contains a simulateError parameter, but I left this outside of the PR for the moment while we validate this approach.

Testing instructions

Confirm that Unit Tests pass. To reproduce more easily, you can make each test run multiple times by adding this to the class:

    override func invokeTest() {
        for _ in 0..<20 {
            super.invokeTest()
        }
    }