Integration Tests: fixed Purchases leak through `PurchasesDiagnos…

…tics` (#2126)

Calling `PurchasesDiagnostics.default` was lazily initializing it:
```swift
public static let `default`: PurchasesDiagnostics = .init(purchases: Purchases.shared)
```

Which was keeping a reference to `Purchases.shared` around.
For most cases this is fine, since `Purchases` would only be initialized
once, but for tests it meant that it was leaking, which is now detected
by #2106.

Tests/BackendIntegrationTests/StoreKitIntegrationTests.swift

@@ -56,7 +56,8 @@ class StoreKit1IntegrationTests: BaseBackendIntegrationTests {
     }
 
     func testPurchasesDiagnostics() async throws {
-        let diagnostics = PurchasesDiagnostics.default
+        let diagnostics = PurchasesDiagnostics(purchases: Purchases.shared)
+
         try await diagnostics.testSDKHealth()
     }