SystemInfo: fix flaky Storefront test (#3411)

Follow up to #3405.

`SystemInfo.storefront` relies on `SKPaymentQueue`, which requires an
`SKTestSession`.
This is why this new test was being flaky. This new implementation
explicitly sets the storefront to fix that.

Tests/StoreKitUnitTests/StorefrontTests.swift

@@ -47,4 +47,14 @@ class StorefrontTests: StoreKitConfigTestCase {
         }
     }
 
+    func testSystemInfoStorefront() async throws {
+        let expected = "ESP"
+        try await self.changeStorefront(expected)
+
+        let systemInfo = SystemInfo(platformInfo: nil, finishTransactions: false)
+        let storefront = try XCTUnwrap(systemInfo.storefront)
+
+        expect(storefront.countryCode) == expected
+    }
+
 }

Tests/UnitTests/Misc/SystemInfoTests.swift

@@ -54,14 +54,12 @@ class SystemInfoTests: TestCase {
         expect(SystemInfo.withReceiptResult(.receiptWithData, sandboxDetector).isSandbox) == false
     }
 
-    func testStorefront() {
-        let storefront = SystemInfo(platformInfo: nil, finishTransactions: false).storefront
-
+    func testStorefrontForUnsupportedPlatforms() throws {
         if #available(iOS 13.0, macOS 10.15, tvOS 13.0, watchOS 6.2, macCatalyst 13.1, *) {
-            expect(storefront?.countryCode) == "USA"
-        } else {
-            expect(storefront).to(beNil())
+            throw XCTSkip("Test only for older platforms")
         }
+
+        expect(SystemInfo.default.storefront).to(beNil())
     }
 
     func testIsAppleSubscriptionURLWithAnotherURL() {